Support authorised user credentials

[v01dXYZ]

Currently we only support service accounts because we directly use google.oauth2.service_account:

python-bigquery-sqlalchemy/sqlalchemy_bigquery/_helpers.py

Lines 45 to 54 in 285913a

	if credentials_path:
	credentials = service_account.Credentials.from_service_account_file(
	credentials_path
	)
	credentials = credentials.with_scopes(SCOPES)
	default_project = credentials.project_id
	elif credentials_info:
	credentials = service_account.Credentials.from_service_account_info(
	credentials_info
	)

A possible solution to that, depending on the credential argument:

• credential_path: directly use load_credentials_from_file from google.auth
• credential_info/credential_base64: Unfortunately the _load_credentials_from_info is private to google.auth, but we can add a utility function to mimic its behaviour for service accounts and authorised users since that's just dispatch for those cases.

google.auth.load_credentials_from_file: https://github.com/googleapis/google-auth-library-python/blob/f3b72e161ec8302251f04b0121d59476a167a729/google/auth/_default.py#L82