Support credential flags

[Temikus]

Currently we don't support passing credentials to ts-bridge directly. It's all done through google-cloud-go library support, so classic application default credentials.

Design:

• App would support additional 2 methods of auth:
  • `--google-json-key" argument and associated env variable allowing to pass in a JSON account key to the app
  • `--google-json-key-string" argument and associated env variable allowing to pass the same account key file as a string
• If set, those flags must override other methods of selecting auth (library already does it so it should be enough to just be careful when piping the options through)

This should require just piping through the options from the lib, described here: https://github.com/googleapis/google-cloud-go#authorization

[Temikus]

Decided against it.