feat: allow NGiNX to correctly process `%2F` special character in path parameters

Referenced APISIX issue: https://github.com/apache/apisix/issues/11810

Description

Currently, APISIX with radixtree_uri_with_parameter router doesn't support %2F in path parameters. That's due to how NGiNX processes the request URIs.

Feature Request

Add a NGiNX configuration directive to allow the correct processing of %2F character in URI. This directive, supposedly called decode_percent_characters would work like that:

• by default is set to on, so that it doesn't change NGiNX default behaviour;
• similarly to merge_slashes directive, it's passed to ngx_http_parse_complex_uri() from src/http/ngx_http_parse.c;
• if set to off, NGiNX doesn't replace percent-encoded characters in request URIs, with their respective decoded values (i.e. %2f doesn't get decoded to / if we set decode_percent_characters off; in nginx.conf);

Additional Notes

I opened a issue some time ago on NGiNX, with the feature proposal and a possible implementation, but it didn't drag attention: https://github.com/nginx/nginx/issues/501.

Questions

• Do you think that this feature makes sense/could be useful?
• Would it effectively solve the issue, or there's something I'm missing in the request processing and routing pipeline? Would that be the right component to introduce this feature?
• How can I test a possible fix myself? Which file(s) do I have to edit and how can I compile?

Start with src/http/ngx_http_parse.c and ngx_http_parse_complex_uri(), comparing the requested behavior with the referenced APISIX issue and NGINX issue #501. Determine how a directive like decode_percent_characters would reach URI parsing and how it could be tested. Done means the default preserves NGINX behavior while off prevents encoded slashes such as %2F from being decoded.