Custom HTTP headers for proxy and faucet API requests

[BubuMVX]

Description

Summary

This change lets users attach extra HTTP headers to proxy traffic (MultiversX API / ProxyNetworkProvider) and to faucet native-auth API calls. That supports authenticated or gated gateways (for example API keys) without changing proxy URLs.

What changed

• --proxy-headers KEY=VALUE [KEY=VALUE ...]: Added wherever add_proxy_arg() is used. Values are parsed in parse_proxy_headers(); each token must contain = or the CLI raises a clear error.
• Global application: Before the subcommand runs, cli.py calls config.set_proxy_headers(...). get_config_for_network_providers() passes those headers to the SDK via NetworkProviderConfig(requests_options={"headers": ...}), so SDK proxy clients pick them up consistently.
• mxpy faucet request: New --api-headers with the same KEY=VALUE form, forwarded to NativeAuthClientConfig(extra_request_headers=...).
• Environment model: env.mxpy.json defaults and MxpyEnv include a proxy_headers field (documented as space-separated KEY=VALUE pairs in get_proxy_headers()).
• utils.parse_headers_list(): Shared parsing for KEY=VALUE lists.
• cli_get: Uses shared add_proxy_arg() so get subcommands get --proxy-headers like the rest of the CLI.
• cli_tokens: TokenManagementController now builds ProxyNetworkProvider with get_config_for_network_providers() so token flows respect the same header configuration.
• Tests: test_proxy_extra_headers asserts custom headers are passed through on ProxyNetworkProvider GETs (mocked requests.Session.get).
• Docs: CLI.md updated with the new flags across affected commands.

Usage examples

mxpy get network-config \
  --proxy https://gateway.example.com \
  --proxy-headers "Api-Key=secret"

mxpy faucet request \
  --chain D \
  --api https://api.example.com \
  --api-headers "Api-Key=secret"

[popenta]

Merged the PR in another branch just to allow the actions to run. We'll review the PR and then we'll merge it into main.

PR

[BubuMVX]

Thanks @popenta !
To fix GitHub Actions on this repo, I suggested some changes in #565.
Feel free to cherry pick, update or reject them.