Add support for client capabilities

[gsabran]

Summary

Add support for client capabilities

Details

• add support for client capabilities (listing roots, sampling)
• improve Client API to make sure a handler is provided when a capability is said to be supported
• add tests

Checklist before merging

• If this is a new feature or a bug fix, I have added tests.

[codecov]

Codecov Report

Attention: Patch coverage is 50.74627% with 33 lines in your changes missing coverage. Please review.

Project coverage is 63.50%. Comparing base (ef2617b) to head (d7e3d17).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
MCPClient/Sources/MCPClient.swift	64.00%	18 Missing ⚠️
MCPClient/Sources/MockMCPConnection.swift	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   62.80%   63.50%   +0.70%     
==========================================
  Files          14       15       +1     
  Lines         941      981      +40     
==========================================
+ Hits          591      623      +32     
- Misses        350      358       +8     

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