Add Middleware handlers to StreamableHttpTransport

[sveneld]

Motivation and Context

This pull request gives a possibility to add different middlewares to StreamableHttpTransport for different approach.

How Has This Been Tested?

Unit tests

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[sveneld]

@chr-hertel please take a look

[chr-hertel]

On the first glance, i feel like it makes it more sophisticated, is it needed tho?
The idea would be that frameworks could register their own middleware and solve auth on their own? or would you rework #194 to use that middleware than? or both eventually i guess?

maybe i'm missing the point here ...

[sveneld]

Yep, that’s the idea. #194 will be rewritten on top of this middleware approach, and it also opens the door for community/custom middlewares without patching core.

[sveneld]

@chr-hertel #221 is a pull request with oAuth implementation based on middleware. I created a new pull request instead of update previous

[sveneld]

@chr-hertel maybe we should go step by step and this pull request will be first part for oauth implementing?