Add HeaderName constants for MCP HTTP headers

[gandhipratik203]

Summary

• Add lowercase http::HeaderName constants for exact MCP HTTP headers while keeping the existing &str constants.
• Add a lowercase Mcp-Param-* prefix constant for prefix matching.
• Switch internal exact header lookups/inserts to the typed constants where useful.

Validation

• cargo test -p rmcp --features client-side-sse,server-side-http --lib transport::common::http_header
• cargo test -p rmcp --lib
• cargo test -p rmcp --features client,transport-streamable-http-client-reqwest,transport-streamable-http-server --lib
• cargo test -p rmcp --features client,transport-streamable-http-client-unix-socket --lib
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features

[gandhipratik203]

Context for this PR: this came out of downstream integration work in ContextForge, where we needed typed HeaderName constants for MCP headers instead of rebuilding them locally.

I checked the contribution notes after opening this and saw the "discuss first" preference. Happy to move this into an issue/discussion first if that is preferred.

This keeps the existing &str constants and only adds typed constants for exact header lookups/inserts.