Add oAuthEndpoint to _oAuthClients cache key

[craigjensen]

Fixes #6308

Description

We cache OAuthClients by appId so changing the endpoint only affects newly created clients that are not already cached. This PR adds the endpoint to the cache key so we can support changing the endpoint at runtime.

Specific Changes

• Added OAuthEndpoint to the cache key for the _oAuthClients cache

Testing

Added tests for

[coveralls]

Pull Request Test Coverage Report for Build 301650

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 78.786%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder/BotFrameworkAdapter.cs	17	47.91%

Totals
Change from base Build 301524:	0.08%
Covered Lines:	25318
Relevant Lines:	32135

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💛 - Coveralls

[jayman-dalal]

The assumption is that we should not be calling these from different threads as we are still caching a static variable. A context switch before this line could result in incorrect endpoint being used if two threads set it to different values. I wonder if the approach is to do better documentation or improve synchronization here? Thoughts?

[craigjensen]

Yes, there will always be a race when using a static like this. As long as we're setting it globally via an FCB I don't think it will matter. For this PR, I wanted to minimize changes and get something that will allow us to change it globally. If we end up needing to set the value on a per-call basis then we'll need another PR to redesign the API and remove the static.