Make Memory and Team an ABC

[victordibia]

Why are these changes needed?

Builds on #5129 to also make base Team class in AgentChat an ABC

Related issue number

Checks

• 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.
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 70.50%. Comparing base (74f411e) to head (a2f827a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5149      +/-   ##
==========================================
+ Coverage   70.48%   70.50%   +0.02%     
==========================================
  Files         174      174              
  Lines       11028    11037       +9     
==========================================
+ Hits         7773     7782       +9     
  Misses       3255     3255              

Flag	Coverage Δ
unittests	70.50% <100.00%> (+0.02%)	⬆️

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