- Adds `extism:host/env::http_headers` to access HTTP response headers
- Adds `PluginBuilder::with_http_response_headers` to enable response
headers from Rust
- Adds `extism_plugin_allow_http_response_headers` to enable response
headers using the C API
TODO:
- [x] Update a PDK to use `extism:host/env::http_headers` so I can test
this