Add OAuthUrl constants and ensure forwarded activities contain a recipient value

[stevengum]

Fixes #1816
Fixes #1997

Description

If the forwarded activity to the skill is missing a recipient, certain methods can throw ReferenceErrors (or in dotnet, NullReferenceExceptions) as Activtiy.recipient is expected (#1997 This PR fixes that and adds a test for this scenario.

The OAuthUrl constants in dotnet were missing from the AuthenticationConstants and GovernmentAuthenticationConstants in botframework-connector. These values have been added in the PR.

Specific Changes

• BotframeworkHttpClient.postActivity adds an empty recipient ({}) to all forwarded activities if the field is missing
• AuthenticationConstants and GovernmentAuthenticationConstants now have OAuthUrl constants

Testing

Added tests

[coveralls]

Pull Request Test Coverage Report for Build 122519

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.06%) to 77.251%

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Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 122201:	-0.06%
Covered Lines:	11852
Relevant Lines:	14615

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💛 - Coveralls