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 76.40449% with 21 lines in your changes missing coverage. Please review.

Project coverage is 79.23%. Comparing base (c26d894) to head (f4614a8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...kages/autogen-core/src/autogen_core/_base_agent.py	61.11%	14 Missing ⚠️
...t/src/autogen_ext/runtimes/grpc/_worker_runtime.py	87.50%	3 Missing ⚠️
...n/packages/autogen-core/src/autogen_core/_agent.py	60.00%	2 Missing ⚠️
...es/autogen-core/src/autogen_core/_agent_runtime.py	50.00%	1 Missing ⚠️
...src/autogen_core/_single_threaded_agent_runtime.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6131      +/-   ##
==========================================
+ Coverage   78.75%   79.23%   +0.48%     
==========================================
  Files         225      225              
  Lines       16589    16666      +77     
==========================================
+ Hits        13064    13205     +141     
+ Misses       3525     3461      -64     

Flag	Coverage Δ
unittests	79.23% <76.40%> (+0.48%)	⬆️

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