Configurable messages endpoint for SSE transport

[edis-uipath]

Add a configurable value for messages endpoint in SseResponseStreamTransport.

Motivation and Context

The SseResponseStreamTransport has a hard-coded /messages endpoint but the MCP spec states that this should be set by the implementation.

When a client connects, the server MUST send an endpoint event
containing a URI for the client to use for sending messages.
All subsequent client messages MUST be sent as HTTP POST requests to this endpoint.

https://spec.modelcontextprotocol.io/specification/2024-11-05/basic/transports/

How Has This Been Tested?

This was tested in the sample application.

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

[halter73]

LGTM.

Long term, we'll probaby want to move this to an options type.