Elastic registry: allow api-key for authentication

[bodin]

Please describe the feature request.
Please allow API keys as an option for authentication into elasticsearch.

Rationale
This allows support for both authentication schemes in elastic and helps remove the need for local users when using SAML/OIDC based user authentication.

Additional context
https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-create-api-key.html

To use the generated API key, send a request with an Authorization header that contains an ApiKey prefix followed by the API key credentials. The credentials are a Base64-encoded string in UTF-8 format that you create by combining the id and api_key with a colon (:). For example:

curl -H "Authorization: ApiKey <credentials>" http://localhost:9200/_cluster/health\?pretty

[jonatan-ivanov]

@bodin This seems like a relatively simple and valuable change to me.
Would you like to help us out and add this functionality? We can help with the changes if you need some.

[bodin]

Happy to provide a PR - I will push it up in the next day or so.

[shakuzen]

New method added to ElasticConfig for configuring API key authentication: apiKeyCredentials.