Return API key name in _authentication response

[justincr-elastic]

Responses to POST /_security/api_key includes id, name, metadata,
api_key (shared secret), and encoded (base64 of id:api_key).

Requests to GET /_security/_authenticate returns data about the user,
but not the API KEY.

When authenticating using an API KEY, return API KEY info map in the
response. The initial feature request asked for 'name'. However, the
request's Authentication header contains 'encoded', so the decoded
'id' will be returned for convenience too.

When authenticating using any other method, API KEY info map is
omitted.

Closes #70306

Example request:

curl -s -X GET -H "Authorization: ApiKey base64(id:api_key)" "localhost:9200/_security/_authenticate?pretty"

Example response:

{
  "username" : "my-username",
  "roles" : [ ],
  "full_name" : null,
  "email" : null,
  "metadata" : { },
  "enabled" : true,
  "authentication_realm" : {
    "name" : "_es_api_key",
    "type" : "_es_api_key"
  },
  "lookup_realm" : {
    "name" : "_es_api_key",
    "type" : "_es_api_key"
  },
  "authentication_type" : "api_key",
  "token" : {
  },
  "api_key" : {
    "id": "jdgWKXwBtdtKDGAHT5OX"
    "name": "my-api-key"
  }
}

[ywangd]

I didn't really look into the client side code. But here are a few things on the server side.

[justincr-elastic]

@elasticmachine update branch

[justincr-elastic]

@elasticmachine update branch

[elasticmachine]

merge conflict between base and head

[justincr-elastic]

@elasticmachine update branch

[elasticmachine]

merge conflict between base and head

[justincr-elastic]

@elasticmachine update branch

[justincr-elastic]

@elasticmachine update branch

[ywangd]

LGTM. (Not sure why my previous disappeared. But I approved already a couple days back.)