[url_launcher] Add native unit tests for Windows (flutter#4156)

Adds a unit test target based on googletest. This is intended to be both a set of unit tests for this plugin, and also a model of changes that can be made to the `flutter create` template for Windows plugins to include better testing out of the box (flutter/flutter#82458).

In addition to the test binary being directly runnable, the integration between CMake, VS, and googletest means that these tests are visible—and runnable—in the VS Test Explorer UI after opening the generated .sln file.

Changes for testing in general:
- Moved the plugin class declaration to a header.
- Moved the C registration API implementation to a separate file.
- Added (opt-in, so it won't affect plugin client builds) plugin CMake rules to download googletest and build a new executable target that builds all the plugin sources, plus gtest and gmock.
- Added a line to the example app CMake rules to enable the unit tests.
- Added a unit test file.

url_launcher-specific changes:
- Wrapped all Win32 calls in a thin class for mockability in unit tests.
- Factored some logic into helpers for better maintainability while I was refactoring anyway.

Note: This unit test is not yet being run by CI. A tools command to run Windows plugin unit tests will be a separate PR.

Part of flutter/flutter#82445

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