Add Enterprise Search Connector Service Account

[jfreden]

This adds a new service account intended to be used by customer backends generating API keys for use with indices populated by connectors.

See drawing below:

• read_security allows the customer backend to query roles.
• manage_own_api_key allows the backend to create api keys and manage the created api keys.

The API keys created will have a role descriptor with read index permissions on the connector populated data index and a DLS query that's fetched from the user role using the query roles API.