Add ability to register Agent instances

[peterychang]

Why are these changes needed?

Nice to have functionality

Related issue number

Closes #6060

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 79.12088% with 19 lines in your changes missing coverage. Please review.

Project coverage is 77.32%. Comparing base (20753ad) to head (edf4414).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...kages/autogen-core/src/autogen_core/_base_agent.py	69.44%	11 Missing ⚠️
...t/src/autogen_ext/runtimes/grpc/_worker_runtime.py	84.61%	4 Missing ⚠️
...src/autogen_core/_single_threaded_agent_runtime.py	90.00%	2 Missing ⚠️
...n/packages/autogen-core/src/autogen_core/_agent.py	50.00%	1 Missing ⚠️
...es/autogen-core/src/autogen_core/_agent_runtime.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6131      +/-   ##
==========================================
+ Coverage   77.08%   77.32%   +0.23%     
==========================================
  Files         197      197              
  Lines       13637    13863     +226     
==========================================
+ Hits        10512    10719     +207     
- Misses       3125     3144      +19     

Flag	Coverage Δ
unittests	77.32% <79.12%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ekzhu]

We can add a few tests for the case of successful registration of instances and then

1. send_message to the registered agent, successfully respond with a reply.
2. publish_message to a topic subscribed by the agent, successfully observed a state change in the agent itself.

[jackgerrits]

Suggested change

	async def init(self, **kwargs: Any) -> None:
	async def bind_id_and_runtime(self, id: AgentId, runtime: AgentRuntime) -> None:

I think we shoud make this a concrete call otherwise it is unclear how or why it would be used