Support NEW VSTS Identity Service Requirement

[yacaovsnc]

Based on commit 70af3d from Git-Credential_manager-for-Windows

Manual test:

1. Compile and install gcm4ml locally
2. Enable debug from .gitconfig
3. Remove cached credentials (on Mac, it's saved in keychain.)
4. Clone a repo hosted by VSTS and observe:

VsoAzureAuthority::populateTokenTargetId
VsoAzureAuthority::createConnectionDataRequest
   validating token
   target identity is add8a40a-597c-43ff-93fa-e0153bc896ab
   parsed identity service url: https://app.vssps.visualstudio.com/Aadd8a40a-597c-43ff-93fa-e0153bc896ab/
   creating access token scoped to 'vso.code_write' for 'add8a40a-597c-43ff-93fa-e0153bc896ab'
   personal access token acquisition succeeded.
SecretStore::writeCredentials

Verify clone is successful and credential is saved properly.

[AlexRukhlin]

Looks good for me

[davidstaheli]

This naming confused me a little, that parseLocationServiceUriFromJson() is used to retrieve the identityServiceUri. Can you add a code comment to explain it?

[yacaovsnc]

You are right, it's a bad name. I am not parsing location "service", I am only parsing the "location" field, I renamed the method to reflect that.

[davidstaheli]

Looks good. Added one comment.