Make the OAuth APIs in .NET testable

[Jeffders]

This PR includes a handful changes to make testing bots using the OAuth APIs and OAuthPrompt easier.

These include:

1. Introduce the IUserTokenProvider interface which includes all of the OAuth APIs on the BotFrameworkAdapter
2. Have BotFrameworkAdapter implement this interface
3. Make all of these methods on BotFrameworkAdapter virtual (so they can be overridden by mock frameworks).
4. Change OAuthPrompt to look for adapter's that implement IUserTokenProvider instead of BotFrameworkAdapter

Additionally, you can now write unit tests for your bot using the TestAdapter and TestFlow:

1. TestAdapter includes a mock, in-memory implementation of IUserTokenProvider
2. Added unit tests for this in-memory implementation
3. Added unit tests for OAuthPrompt that test the entire login flow both with and without a magic code response.

[coveralls]

Pull Request Test Coverage Report for Build 48466

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 54 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.7%) to 72.167%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder/Adapters/TestAdapter.cs	54	79.86%

Totals
Change from base Build 48458:	2.7%
Covered Lines:	3744
Relevant Lines:	5188

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💛 - Coveralls

[tomlm]

[tomlm]

Hey jeff, can we make sure this is applied to js side too?