Add TfsPullRequestSettings overload which uses environment variables from Build

[pascalberger]

Add TfsPullRequestSettings overload which uses available environment variables from TFS / Azure DevOps build (BUILD_REPOSITORY_URI, SYSTEM_PULLREQUEST_PULLREQUESTID)

[yansklyarenko]

Could you please clarify how this new constructor should work?

As far as I understand:

• it should only accept ITfsCredentials as a parameter
• it should assert the value of repository Url it gets from the env for null
• it should NOT assert either source branch or pull request ID values (as far as I can see, it is handled during the construction of the TfsPullRequest object)

Correct?

[pascalberger]

There are multiple things which can be done here:

1. Add an Ctor overload public TfsPullRequestSettings(ITfsCredentials credentials) which reads BUILD_REPOSITORY_URI and SYSTEM_PULLREQUEST_PULLREQUESTID environment variable, asserts that they are set and contain a value and the sets this.RepositoryUrl and this.PullRequestId.
   This helps to construct a settings object in a Cake script which is executed in a TFS / Azure DevOps build.
2. Create a static method TfsPullRequestSettings.UsingTfsBuildOAuthToken() which returns an instance using the new ctro from the previous point by passing an instance of TfsAuthenticationOAuth to it with the value from the SYSTEM_ACCESSTOKEN environment variable.
   This helps to construct a settings object in a Cake script which is executed in a TFS / Azure DevOps build which has an OAuth token provided to the build.
3. Add an alias TfsPullRequestUsingTfsBuildOAuthToken(this ICakeContext context) which uses settings created by the static method from the previous point.
   This makes it even simpler to get access to the pull request in a TFS / Azure DevOps build which has an OAuth token provided to the build.