Built-in agent API Key creation role

**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**

https://github.com/elastic/apm-server/pull/11881#issuecomment-1779548730