Add client credentials authentication for SSE and streamable http mcp server connections

[BartMemelink]

• update readme with environment variables descriptions
• add client credential option for authentication instead of a fixed token

note: the token url is now provided from the environment variables. The solution could be expanded later on with discovery from well-known endpoints according to the rfc9728 spec

[sparfenyuk]

Looks great! Can you please revise the solution since it has some conflicts as of now. Also, consider command-line arguments in addition to env vars.

[BartMemelink]

Hi @sparfenyuk I do agree the command-line arguments are a better fit. I have updated the solution and also resolved the conflicts. Most appreciated if you can have another look!

[sparfenyuk]

Looks good. Fix the build checks.

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/mcp_proxy/__main__.py	50.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (69.23%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   87.54%   87.29%   -0.25%     
==========================================
  Files          11       11              
  Lines        1092     1102      +10     
  Branches       64       64              
==========================================
+ Hits          956      962       +6     
- Misses        117      121       +4     
  Partials       19       19              

Files with missing lines	Coverage Δ
src/mcp_proxy/sse_client.py	69.23% <100.00%> (+2.56%)	⬆️
src/mcp_proxy/streamablehttp_client.py	69.23% <100.00%> (+2.56%)	⬆️
tests/test_cli_arguments.py	94.11% <100.00%> (ø)
src/mcp_proxy/__main__.py	44.82% <50.00%> (+0.30%)	⬆️

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