feature: Add parse_der_cert and parse_der_priv_key functions

[devicenull]

This is the corresponding Lua side of openresty/lua-nginx-module#2278 and openresty/stream-lua-nginx-module#336 and should not be merged until those make it in.

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

[devicenull]

Tests failing is entirely expected right now, they won't pass until the lua-nginx-module changes are merged

[zhuizhuhaomeng]

This is the corresponding Lua side of openresty/lua-nginx-module#2278 and should not be merged until that makes it in.

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

You also need to check if the CI is passed.

[devicenull]

This is the corresponding Lua side of openresty/lua-nginx-module#2278 and should not be merged until that makes it in.
I hereby granted the copyright of the changes in this pull request to the authors of this lua-resty-core project.

You also need to check if the CI is passed.

Tests here cannot pass until the C side of this is in. Error message is currently:

2024/01/17 20:47:55 [error] 31953#0: *4 lua entry thread aborted: runtime error: ./lib/ngx/ssl.lua:114: /opt/luajit21/lib/libluajit-5.1.so.2: undefined symbol: ngx_http_lua_ffi_parse_der_cert" (req 0)

This functionality was only added here and here and has not been merged yet.

[zhuizhuhaomeng]

merged with small modifications.