Allows one user or service account to impersonate another using iamcredentials api

its already implemented or pending for several other languages:

• python: https://github.com/googleapis/google-auth-library-python/blob/master/google/auth/impersonated_credentials.py#L17

• java: https://github.com/googleapis/google-auth-library-java/blob/master/oauth2_http/java/com/google/auth/oauth2/ImpersonatedCredentials.java

• golang: https://go-review.googlesource.com/c/oauth2/+/143957

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here's what i've done so far:

• impersonatedclient.ts
• usage