Update _ilm policy CRUD APIs to be behind _ilm/policy namespace

[talevy]

there are multiple cluster-level APIs for ILM that reside behind the _ilm URL path.

Endpoints like:

• _ilm/start
• _ilm/stop
• _ilm/status

The above conflict with the get-policy API _ilm/ where a policy name can be
"start", "stop", or "status".

This can be fixed by moving the CRUD APIs to be behind the _ilm/policy/ path prefix.

So

GET/DELETE/PUT _ilm/policy/

[elasticmachine]

Pinging @elastic/es-core-infra

[colings86]

Note that the plan was to move the policy CRUD APIs (Put, get, delete lifecycle policy) to the _ilm/policy namespace. The stop start and status APIs should stay where they are