Allow custom header injection for HTTP requests (#591)

[Sadeequ]

Closes #591

---

This PR implements support for arbitrary headers on RPC HTTP requests, addressing issue #591.

Changes include:

• Added parsing in TypeScript and Go configs, CLI flags (), and environment variable support.
• Updated RPC clients (Go builder and transport) to include headers on requests.
• Added unit tests verifying header parsing and propagation in both languages.
• Extended all relevant CLI commands to expose the new flag and honor config/env precedence.
• Updated documentation in CLI and RPC_FALLBACK guides.
• Added tests for header precedence and network probe behavior.

The feature allows users to inject custom authorization or proxy headers, useful for enterprise firewalls. The implementation is backward compatible and covered by new tests.

Please review and merge when ready. Thanks!

[dotandev]

fix conflicts, please.

[dotandev]

fix the newest conflicts, please.