Pass request builder to constructor to support custom headers

[renxiawang]

Adding support for custom request headers for SSE transport

Motivation and Context

Close #68

How Has This Been Tested?

This change simply injects a HttpRequest.Builder to HttpClientSseClientTransport. Existing HttpClientSseClientTransportTests passed. I considered adding a test to mock the http client and verify a request is sent with custom headers within the unit, but it seems testing too much of the internal logic of the unit, not the input/output behavior. Let me know what you think.

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 (where is the style guideline?)
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[tzolov]

Thank you @renxiawang ! will review it shortly