feat: better http server support

[4t145]

1. allow user get extensions in tool call. To support feat: enable tool access to HTTP request headers #175
2. allow user serve sse service without initialization. To support this

Motivation and Context

How Has This Been Tested?

Breaking Changes

Now the peer_info in Peer could be uninitialized. This allows users serve a transport without initialization.

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