[xDS] A97 - JWT token file call creds
@ejona86 Could you please review this PR when you get a chance? Thanks!
@ejona86 PR ready for second round
@ejona86 kindly reminder about this forgotten piece :)
@ejona86 - I applied the fixes from your latest comments.
I had to rebase the PR due to local compilation issues. However, all new changes can be found starting from commit 6: Default implementation for newCallCredentials
@ejona86 - newest changes published
@Zgoda91, I fixed the Bazel build. You'll want to pull from your branch to pull in my commit before making further changes (or you'll need to merge later).