Enabling MSAL to read federation metadata

[trwalke]

Enabling MSAL to read federation metadata form a file provided by the developer.
Correcting MessageID character casing.

Partial fix for #2152

[neha-bhargava]

Minor comments and a question, otherwise looks good.

[pmaytak]

This is out of scope for this issue, and I'm not super familiar with this WS-Trust - we seem to get the metadata on every request (either file or from web). Is there a benefit to cache metadata or does it have to be retrieved each time?

[bgavrilMS]

LGTM, but we should also add a unit test around AcquireTokenByIntegratedWindowsAuth, similar to

microsoft-authentication-library-for-dotnet/tests/Microsoft.Identity.Test.Unit/RequestsTests/IntegratedWindowsAuthUsernamePasswordTests.cs

Line 250 in 52dbbcc

public async Task AcquireTokenByIntegratedWindowsAuthTestAsync()

. This will at least ensure that MSAL does not make the HTTP call to get the mex document.

[bgavrilMS]

Minor comment changes + needs another unit test.