chore(actors): remove pkg/actor usage from agentrm

[stoksc]

Description

This is the product of 3 stacked PRs.

The first rewrote the agents and agent actors as non-actors. I started by refactoring the interactions
between resource managers, resource pools and agents; the agent management subsystem can
be instantiated without a dependency on resource pools, it has a clear API for external consumers
and the resource manager and pools are wired up to use this API.

The second and third wrote the resource pool and manager, respectively, without pkg/actor. Once
the interactions between agents and these were fixed by the first, they mostly just fell into place;
for each message that the actor used to handle, it got a public method for callers instead.

This is the last major change required to remove pkg/actor.

Test Plan

Covered by automated tests and "make sure agentrm works well".

Commentary (optional)

Woohoo.

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

DET-9656

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