* merge RefreshingManagedChannel & SafeShutdownManagedChannel into ChannelPool.

The eventual goal is to allow channel pool to safely add remove channels. The code has been refactored roughly as:
- SafeShutdownManagedChannel is now ChannelPool.Entry and ReleasingClientCall
- RefreshingManagedChannel has been merged into ChannelPool as a pair of functions scheduleNextRefresh & refresh

* migrate SafeShutdownManagedChannel tests

* migrate old tests and remove RefreshingManagedChannel and SafeShutdownManagedChannel

* fix test

* address feedback

* fix race condition on refresh()

* fix warnings in test

* Update gax-grpc/src/test/java/com/google/api/gax/grpc/ChannelPoolTest.java

Co-authored-by: Chanseok Oh <[email protected]>

* Update gax-grpc/src/test/java/com/google/api/gax/grpc/ChannelPoolTest.java

Co-authored-by: Chanseok Oh <[email protected]>

* handle race condition

* Update gax-grpc/src/main/java/com/google/api/gax/grpc/ChannelPool.java

Co-authored-by: Chanseok Oh <[email protected]>

Co-authored-by: Chanseok Oh <[email protected]>

🤖 I have created a release \*beep\* \*boop\*
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* feat: pass a CallCredentials to grpc-java for DirectPath

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Follow-up to: #1589 (comment)

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### Release Notes

<details>
<summary>gradle/gradle</summary>

### [`v7.3.3`](https://togithub.com/gradle/gradle/releases/v7.3.3)

This is a patch release for Gradle 7.3.

It fixes the following issues:

-   [#&#8203;19360](https://togithub.com/gradle/gradle/issues/19360) Upgrade checks to Log4j 2.17.0

We recommend users upgrade to 7.3.3 instead of 7.3.

Given the context of the Log4Shell vulnerability, make sure you take a look at [our blog post](https://blog.gradle.org/log4j-vulnerability) on this topic.

#### Upgrade Instructions

Switch your build to use Gradle 7.3.3 by updating your wrapper:

    ./gradlew wrapper --gradle-version=7.3.3

See the [Gradle 7.x upgrade guide](https://docs.gradle.org/7.3.3/userguide/upgrading_version\_7.html#changes\_7.3) to learn about deprecations, breaking changes and other considerations when upgrading to Gradle 7.3.3.

#### Reporting Problems

If you find a problem with this release, please file a bug on [GitHub Issues](https://togithub.com/gradle/gradle/issues) adhering to our issue guidelines.
If you're not sure you're encountering a bug, please use the [forum](https://discuss.gradle.org/c/help-discuss).

### [`v7.3.2`](https://togithub.com/gradle/gradle/releases/v7.3.2)

This is a patch release for Gradle 7.3.

It fixes the following issues:

-   [#&#8203;19300](https://togithub.com/gradle/gradle/issues/19300) Mitigations for log4j vulnerability in Gradle builds
-   [#&#8203;19257](https://togithub.com/gradle/gradle/issues/19257) Incremental java compilation fails when renaming classname with $ character

We recommend users upgrade to 7.3.2 instead of 7.3.

Given the context of the Log4Shell vulnerability, make sure you take a look at [our blog post](https://blog.gradle.org/log4j-vulnerability) on this topic.

#### Upgrade Instructions

Switch your build to use Gradle 7.3.2 by updating your wrapper:

    ./gradlew wrapper --gradle-version=7.3.2

See the [Gradle 7.x upgrade guide](https://docs.gradle.org/7.3.2/userguide/upgrading_version\_7.html#changes\_7.3) to learn about deprecations, breaking changes and other considerations when upgrading to Gradle 7.3.2.

#### Reporting Problems

If you find a problem with this release, please file a bug on [GitHub Issues](https://togithub.com/gradle/gradle/issues) adhering to our issue guidelines.
If you're not sure you're encountering a bug, please use the [forum](https://discuss.gradle.org/c/help-discuss).

### [`v7.3.1`](https://togithub.com/gradle/gradle/releases/v7.3.1)

This is a patch release for Gradle 7.3.

It fixes the following issues:

-   [#&#8203;19058](https://togithub.com/gradle/gradle/issues/19058) Consider reverting breaking change about test configuration
-   [#&#8203;19067](https://togithub.com/gradle/gradle/issues/19067) Fix multiple annotation processing issues discovered by Micronaut

We recommend users upgrade to 7.3.1 instead of 7.3.

#### Upgrade Instructions

Switch your build to use Gradle 7.3.1 by updating your wrapper:

    ./gradlew wrapper --gradle-version=7.3.1

See the [Gradle 7.x upgrade guide](https://docs.gradle.org/7.3.1/userguide/upgrading_version\_7.html#changes\_7.3) to learn about deprecations, breaking changes and other considerations when upgrading to Gradle 7.3.1.

#### Reporting Problems

If you find a problem with this release, please file a bug on [GitHub Issues](https://togithub.com/gradle/gradle/issues) adhering to our issue guidelines.
If you're not sure you're encountering a bug, please use the [forum](https://discuss.gradle.org/c/help-discuss).

</details>

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