Add OAuth2 support

[mem]

Recent versions of BBE have added support for OAuth2. You have to pass a secret that is used to contact a server which in turn provides a secret that can be used to access another server.

Do note that we need to make sure we are not asking for the secret every time we run a check. We need to leverage the fact that the provided token is valid over an extended period of time.

Tasks

• Upgrade BBE (not required)
• Add support for the new options in the proto files (Add oauth2 support to HTTP checks #377)
• Pass options to BBE (Add oauth2 support to HTTP checks #377)
• Add support for the new options in the API
• Add support for the new options in the plugin
• Update BBE / prometheus/common to cache token

[mem]

We need to figure out if BBE will cache the token / pay attention to expiration. Because of the way BBE works, I don't see why it would out of the box.