feat: add auth support for GCP access tokens (#574)

Introduce choice "accesstoken" for auth_kind for modules
Introduce optional access_token string param that falls back to
  GCP_ACCESS_TOKEN env var

plugins/modules/gcp_sql_user.py

@@ -87,6 +87,7 @@ options:
     - application
     - machineaccount
     - serviceaccount
+    - accesstoken
   service_account_contents:
     description:
     - The contents of a Service Account JSON file, either in a dictionary or as a
@@ -101,6 +102,10 @@ options:
     - An optional service account email address if machineaccount is selected and
       the user does not wish to use the default email.
     type: str
+  access_token:
+    description:
+    - An OAuth2 access token if credential type is accesstoken.
+    type: str
   scopes:
     description:
     - Array of scopes to be used