Feature request : OpenID Connect : add support for "Client Credentials" and "Ressource Owner Password" grant type

**Describe the feature**:

Hello
Thanks to @jkakavas, since version 7.2 elasticsearch [does support OpenID (oauth2) as authentication method](https://www.elastic.co/guide/en/elasticsearch/reference/master/oidc-guide-authentication.html). Two OpenID flows are currently implemented : ```Authorization Code``` and ```Implicit```

This is great. It work perfectly...As long as you are an user using Kibana. These two flows are not designed for scripts performing queries on Elastic.

OpenID also support ```machine to machine``` logins, using other flows (```Client Credentials``` and ```Ressource Owner Password```).

It would be nice to implement those into Elastic. This way, it would be possible to authenticate service accounts on elastic using an OpenID server, instead of managing those accounts locally.

Would it be possible to integrate this in the future ?