[Extensions] Core-side API to get auth token for service account

[stephen-crawford]

To implement service accounts, it needs to be possible to fetch the corresponding auth token from the Security Plugin. This means that core should be able to use a transport API to request the auth token corresponding to the service account. The auth token will contain information relating to the account's roles and identity.

The completion of this issue will be a PR that adds a transport API to core which can connect to the Security plugin and request an auth token for a given service account. The generation of the token and the plugin-side handling of the token are outside the scope of this issue. This issue is best completed as a follow-up to #2611, which adds the base Security Plugin side of the API functionality.

[stephen-crawford]

This is being addressed in opensearch-project/OpenSearch#7452 since we can implement the interface in the Security Plugin and then use it to fetch the tokens for the extensions.