feat(build): build wasmx as static or dynamic module

[locao]

Summary

This change makes it possible to build ngx_wasm_module as a dynamic Nginx module.

It adds the new flag --//:wasmx_module, which accepts dynamic and static options. Currently dynamic is the default value.

There is a new feature to include the load_module directive in nginx.conf when dynamic module is in use and wasm = on. It will search for the shared object in the path defined by the environment variable KONG_DYNAMIC_MODULES_PATH, the nginx build prefix and then in /usr/local/openresty/nginx/modules (the default installation path).

Checklist

• [ ] The Pull Request has tests
• [ ] There's an entry in the CHANGELOG
• [ ] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• Build scripts changed.

Issue reference

KAG-664