Support for providing token_url as a file in http_config.oauth2 #4759
Description
Relevant docs: https://prometheus.io/docs/alerting/latest/configuration/#oauth2
The same way that it is useful to have the optionality to provide a Client Secret via client_secret or client_secret_file, it would be useful to have a token_url_file field to support providing a token_url value via file.
This is particularly useful for Helm deployments of Alertmanager that can only resolve the token_url at deployment time.