Built-in agent API Key creation role

[axw]

Is your feature request related to a problem? Please describe.

As a non-superuser, if I want to create agent API Keys then I must create a custom Elasticsearch role as described by https://www.elastic.co/guide/en/apm/guide/current/api-key.html#create-api-key-user. This is increases the barrier to getting started.

Describe the solution you'd like

I would like to have a predefined role which can create agent API Keys. This should either be a role dedicated to creating agent API Keys, a broader role such as "editor", or perhaps both.

Describe alternatives you've considered

None

Additional context

elastic/apm-server#11881 (comment)