[#6596] TeamsChannelData need OnBehalfOf

[ceciliaavila]

Fixes #6596

Description

This PR adds the OnBehalfOf property to send messages via bots on behalf of another user in Teams.
It also adds unit tests to cover the new code.

Specific Changes

• Added the OnBehalfOf property in the TeamsChannelData class.
• Added the TeamsGetTeamOnBehalfOf method to return the current activity's OnBehalfOf list.

Testing

The following image shows the message of the bot on behalf of another user in the Teams channel.

The following image shows the related unit test passing.

[coveralls]

Pull Request Test Coverage Report for Build 345844

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.008%) to 79.096%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetNextViableTime.cs	1	90.91%
/libraries/AdaptiveExpressions/BuiltinFunctions/GetPreviousViableTime.cs	1	90.91%
/libraries/Microsoft.Bot.Streaming/Payloads/StreamManager.cs	1	90.0%
/libraries/Microsoft.Bot.Streaming/Transport/NamedPipe/NamedPipeClient.cs	1	71.43%
/libraries/Microsoft.Bot.Builder/Teams/TeamsActivityExtensions.cs	4	78.26%

Totals
Change from base Build 344537:	-0.008%
Covered Lines:	25817
Relevant Lines:	32640

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💛 - Coveralls

[BruceHaley]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll