Adding Custom Header

[BalamuraliKirhsnan]

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We are using google Oauth library to get idTokenCredentials. In our organization we have to use forward proxy to connect to googleapi.com/token. The other system which does forward proxy needs additional header and am not finding option to add as library does it all.

Also, i looks for JWT token claims which will help us to prepare JWT ourself and pass it on request to get the IDToken.