Replace 3rd party and openresty scripts modules with njs or remove

[rikatz]

Some 3rd party modules and some openresty scripts can be replaced with njs. This would simplify the compilation of our binaries and reduce a bit our dependency on openresty.

The proposed replacements are:

• more_set_headers and more_clear_headers (https://github.com/openresty/headers-more-nginx-module?tab=readme-ov-file#more_set_headers) can be replaced with an njs script to set headers during response time. This can also be kept, if there's a binary compilation. This module is supported by crossplane, and if we have a binary of it is worth keeping
• set_escape_uri - We can do this encoding via js
• auth_digest - Should be verified. Also we should check who is using it, the module seems not being maintained
• substitution doesn't seem to be used and can be removed
• external auth can be rewriten to njs, may have some bigger effort
• auth cache key sets a base64 variable, may be rewritten (need to check)
• server redirects can be rewritten for njs

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