Update devcontainer.json with copilot extension

[gagb]

Why are these changes needed?

GitHub copilot is a common and very useful extension. We should just add it to our devcontainer!
I currently have to manually install it every time I start a new code space.

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f8ccf8) 35.02% compared to head (cbc4c1d) 35.02%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1556   +/-   ##
=======================================
  Coverage   35.02%   35.02%           
=======================================
  Files          43       43           
  Lines        5207     5207           
  Branches     1211     1211           
=======================================
  Hits         1824     1824           
  Misses       3231     3231           
  Partials      152      152           

Flag	Coverage Δ
unittests	35.02% <ø> (ø)

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[qingyun-wu]

LGTM