Add support to custom extensions in streamable http client

[OS-anasantos]

Motivation and Context

At the moment when using a private gateway, streamablehttp_client will return error ConnectError('[SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:1000)'). This is because it needs sni_hostname. According to this httpx documentation, not only "headers" must have the hostname value, but custom extensions must be sent (extensions = {"sni_hostname": "www.encode.io"})

Unfortunately, streamable_http client does not allow sending custom extensions.

How Has This Been Tested?

• Added unit tests
• Added a small client example, tested it with MCP_SERVER_HOSTNAME = mcp.deepwiki.com (to setup the private gateway I used chisel)

Breaking Changes

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