pack agenthost as tool

[rysweet]

Why are these changes needed?

convenience - allows to just run "agenthost"

dotnet pack --no-build --configuration Release --output './output/release' -bl\n
dotnet tool install --add-source ./output/release Microsoft.AutoGen.AgentHost
agenthost 

Related issue number

closes #5646

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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.95%. Comparing base (3a239c5) to head (2600f3f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5647   +/-   ##
=======================================
  Coverage   68.95%   68.95%           
=======================================
  Files         242      242           
  Lines       12435    12435           
  Branches      242      242           
=======================================
  Hits         8574     8574           
  Misses       3672     3672           
  Partials      189      189           

Flag	Coverage Δ
unittests	68.95% <ø> (ø)

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