Parametrize not only authorized_user_filename but also credentials_filename

[slmtpz]

Is your feature request related to a problem? Please describe.
While instantiating a client using gspread.auth.oauth(), the latest release v3.7.0 does not parameterize the filenames. However, in master, I see that authorized_user.json file is parametrized already. I think it's for the best the file used by auth flow, namely credentials.json is parametrized among oauth() inputs.

Describe the solution you'd like
Allow the users to pass credentials_filename and use it during Google browser auth flow. Pass the file path to downstream functions that need it.

Describe alternatives you've considered
Not an alternative but maybe while there, it could be nice if filenames are renamed to filepaths as for instance DEFAULT_CREDENTIALS_FILENAME is not actually a file name but a file path.

[slmtpz]

I can pick this up if the work sounds good

[burnash]

@slmtpz sounds good. Please go ahead.

Regarding the "filename", I know its arguable but I'd leave it as it is to stay consistent with google-auth which uses the word "filename" for path to files parameters.