…_plugin::CurrentPlugin` in the current scope (#773)

think there's a typo for this block when the `http` feature is disabled

… out on plugin call (#776)

Fixes #775

134/EXIT_SIGNALED_SIGABRT was chosen as the wasmtime CLI exits with it
executes `unreachable`.

…env::http_request` (#774)

- 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

)

Fixes #739