Add capability to impersonate a service account.

This feature is idential to the capability existing in

• google-auth-python
• google-auth-java: ImpersonatedCredentials.java

the original PR against oauth2 was abandoned on suggestion it gets added to this repo
ref: golang/oauth2#336

as a folwlup, i created #337 againts this repo but on request from the maintainers here back then, they would implement it separately somehow. Its not implemented yet so are we ok to reopen the the PR 337?

If so, the sample usage looks like
https://gist.github.com/salrashid123/7994be642c466844f85e2f2c68a5dec9