feature: add precontent_by_lua directives

[HanadaLee]

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.

Since balancer_by_lua_block does not support consocket, getting upstream peers from the outside is usually performed in the access_by_lua_*. (refer to https://github.com/openresty/lua-nginx-module?tab=readme-ov-file#balancer_by_lua_block)

However, since the access phase is mainly responsible for access control, many subrequests usually skip the access phase, resulting in subrequests being unable to get upstream peers in the access phase. The current solution is to identify sub-requests and then move the relevant processing logic forward to rewrite_by_lua_*. But it is ultimately an unreasonable solution, as it inappropriately utilizes the rewrite or access phase to provide functionality that does not belong to them.

In nginx, there is actually a precontent phase after the post_access phase. Initially, nginx named this phase NGX_HTTP_TRY_FILES_PHASE, and it was a dedicated phase for the try_files module (other module handlers could not be registered). As nginx's functionality expanded, it was renamed NGX_HTTP_PRECONTENT_PHASE, which also allowed the registration of new module handlers.

By injecting Lua code into the precontent phase, we can implement proxy/upstream-related preparation functions, including upstream header rewriting, proxy authentication, proxy URI rewriting, and upstream information retrieval (such as obtaining the upstream peer list via DNS). Furthermore, the precontent phase will not be bypassed in sub-requests (one of the biggest problems with registering non-access control related functions in the access phase).

This PR added 3 directives:

precontent_by_lua_block
precontent_by_lua_file
precontent_by_lua_no_postpone

The PR for lua-resty-core: openresty/lua-resty-core#519

close #2253

[4O4-Not-F0und]

Just tried and works perfectly. really helpful, thanks bro

[zhuizhuhaomeng]

move the code after NGX_HTTP_LUA_CONTEXT_PROXY_SSL_VERIFY

[zhuizhuhaomeng]

change to your email

[zhuizhuhaomeng]

don't keep the dead code copy from other file