feat: add HTTP header support for method arguments

[klaidliadon]

Summary

• Add support for routing method arguments to HTTP headers using MethodArgument.Location from the webrpc schema
• Client sends header-located arguments as HTTP headers and excludes them from the JSON body
• Server extracts header arguments from request headers with type-specific parsing (string, number, boolean)

Changes

• client.go.tmpl: Header inputs set as HTTP headers via _headers Record; body-only inputs sent via destructuring; works for both streaming and non-streaming paths
• server.go.tmpl: Add reqHeaders parameter to serve*Rpc and dispatch*Request; build args object combining header + body values with case-insensitive header lookup and type coercion
• _examples/node-ts-header/: New example with RIDL schema, Node HTTP server, and 6 vitest tests
• _examples/node-ts, sse: Regenerated with updated templates

Test plan

• node-ts-header example: 6/6 tests pass (ping, header auth, missing token, multiple headers, all-headers, unauthorized)
• Regenerated examples produce expected output