port: [#4632] Support Federated Identity Credential

[sw-joelmut]

Fixes #4632

Ported from microsoft/botbuilder-dotnet#6838

Description

This PR ports the Federated Credentials from DotNet's repo. The implementation is similar except in the fetchExternalToken, since it requires the Audience value to be provided, by default it uses 'api://AzureADTokenExchange'.

Specific Changes

• Retrieving the clientAssertion token functionality has being extracted from the ManagedIdentity FIC sample.
• Added FederatedAppCredentials class, it gathers a token from a Managed identity resource, and provides the clientAssertion to MSAL.
• Added FederatedServiceClientCredentialsFactory class, so it can be used from CloudAdapter config.

Bot usage example

Testing

As we FIC isn't available for third-party apps, we encountered the following error (same error when testing DotNet's implementation).

[coveralls]

Pull Request Test Coverage Report for Build 11235721258

Details

• 12 of 33 (36.36%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 85.242%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botframework-connector/src/auth/federatedAppCredentials.ts	5	15	33.33%
libraries/botframework-connector/src/auth/federatedServiceClientCredentialsFactory.ts	4	15	26.67%

Totals
Change from base Build 11164635173:	-0.1%
Covered Lines:	20444
Relevant Lines:	22942

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💛 - Coveralls