[Cherry-Pick to 4.13] Add ShowSignInLink to OAuthPromptSettings (#5906)

[mrivera-ms]

• Add ShowSignInLink to OAuthPromptSettings

• Refactor tests

Fixes #5887

[coveralls]

Pull Request Test Coverage Report for Build 276934

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 75 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.01%) to 78.646%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder/DelegatingTurnContext.cs	1	88.24%
/libraries/Microsoft.Bot.Builder.Dialogs.Adaptive/Recognizers/EntityRecognizers/TextEntityRecognizer.cs	1	85.71%
/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/DatetimePrompt.cs	1	77.5%
/libraries/Microsoft.Bot.Builder/Adapters/TestFlow.cs	2	78.32%
/libraries/Microsoft.Bot.Builder.Dialogs/Memory/DialogStateManager.cs	4	78.76%
/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/OAuthPrompt.cs	22	86.22%
/libraries/Microsoft.Bot.Builder/Adapters/TestAdapter.cs	44	79.21%

Totals
Change from base Build 276431:	0.01%
Covered Lines:	18058
Relevant Lines:	22961

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💛 - Coveralls