[AIP-4117]: Support credential_source.env similar to .file

[f0o]

In some cases you have your OIDC JWT presented as Env-Var such as in numerous CI/CD systems.

Instead of writing the OIDC JWT onto a temporary file it would be beneficial if ADC could accept the credential_source as environment variable - or straight up as embedded content.

Perhaps something like:

{
 "type": "external_account",
 ...
 "credential_source": {
  "env": "JWT_TOKEN_VARIABLE"
 }
}