Add server support

[gsabran]

Summary

Add server support

Details

• Add server support
• Refactor the connection interface to share it between client and server
• Refactor the connection tests to share them between client and server
• Add an example of an executable server over stdio
• Lot of small improvements that became apparent through this large change

Outstanding todos:

• Add support for cancelling requests
• Add support for sending progress
• Add more tests for the client and server interfaces (not the connection layer that is well tested)
• Possibly simplify the ergonomic for describing capabilities and building request handlers.

Checklist before merging

• I have added tests.

[codecov]

Codecov Report

Attention: Patch coverage is 54.05743% with 368 lines in your changes missing coverage. Please review.

Project coverage is 56.97%. Comparing base (4cbe50a) to head (40c1c9d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
MCPServer/Sources/MCPServer.swift	0.00%	176 Missing ⚠️
...ver/Sources/Convenience/Schemable+extensions.swift	0.00%	75 Missing ⚠️
MCPTestingUtils/Sources/MockTransport.swift	52.94%	16 Missing ⚠️
MCPTestingUtils/Sources/TestUtils.swift	88.97%	15 Missing ⚠️
MCPClient/Sources/MCPClient.swift	75.00%	14 Missing ⚠️
MCPInterface/Sources/mcp_interfaces/JSON.swift	56.66%	13 Missing ⚠️
MCPInterface/Sources/Interfaces.swift	25.00%	12 Missing ⚠️
MCPInterface/Sources/MCPConnection.swift	87.50%	7 Missing ⚠️
.../Sources/mcp_interfaces/Interface+extensions.swift	94.16%	7 Missing ⚠️
MCPServer/Sources/DataChannel+stdio.swift	0.00%	7 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   63.50%   56.97%   -6.54%     
==========================================
  Files          15       26      +11     
  Lines         981     1571     +590     
==========================================
+ Hits          623      895     +272     
- Misses        358      676     +318     

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