Clients should support credentials_file via client_options

[busunkim96]

Clients should allow credentials_file to be specified through client_options. This credentials_file can be a service account json file or an authorized user file.

Existing methods on the client from_service_account_json and from_service_account_file will be kept. https://googleapis.dev/python/texttospeech/latest/gapic/v1/api.html#google.cloud.texttospeech_v1.TextToSpeechClient.from_service_account_file

from google.cloud import texttospeech

client = texttospeech.TextToSpeechClient(
    client_options={"credentials_file": "path/to/credentials.json"}
)

Internal link: go/extensible-client-options-exit

[busunkim96]

There's a function in the auth library the reads credentials from a file that we can use for this

https://github.com/googleapis/google-auth-library-python/blob/master/google/auth/_default.py#L72