feat(agent): http proxy support

[icycodes]

Changes:

• Add support for http proxy. The proxy can be configured using:
  • environment variables ( http_proxy, https_proxy, no_proxy ) more detail
  • in ~/.tabby-client/agent/config.toml, override environment variables

    [proxy]
    url = "http://127.0.0.1:1087"
    authorization = "Basic dXNlcjpwYXNz"
    

Proxy config in IDE (VSCode) is not supported for now. This is planned to be supported in a future release.