[Core] Unify function decorators for function calling, reply, termination and hook functions

[davorrunje]

Why are these changes needed?

Add function decorators for registering functions for reply, termination and hook and unify API

Describe the solution you'd like

Add the following function decorators:

@agent.register.for_execution
@agent.register.for_llm
@agent.register.for_reply
@agent.register.for_termination
@agent.register.for_hook

Related issue number

Closes #1214

Checks

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

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (2e519b0) 30.63% compared to head (20ba740) 41.20%.

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	85.71%	3 Missing and 3 partials :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1215       +/-   ##
===========================================
+ Coverage   30.63%   41.20%   +10.57%     
===========================================
  Files          32       32               
  Lines        4365     4402       +37     
  Branches     1015     1074       +59     
===========================================
+ Hits         1337     1814      +477     
+ Misses       2932     2450      -482     
- Partials       96      138       +42     

Flag	Coverage Δ
unittests	41.14% <85.71%> (+10.55%)	:arrow_up:

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