feature parity with legacy ytproxy

[paolafrancesca]

• legacy ytproxy accepts a PREFIX_PATH env variable for special deployment case where the proxy is hosted behind a path rather on a separate domain
• legacy ytproxy filters out more sensitive headers to be forwarded

[paolafrancesca]

@FireMasterK , I was rewriting the response in nginx with lua, to have the urls from proxy be prepended by the proxy path. but it was very slow and produced a lot of buffering, so having support for it directly in the proxy would be awesome.

please, let me know what do you think of this feature

thanks :)

[paolafrancesca]

I want this to be a configurable non-default feature at build time.

will you plan to release a separated docker image with that build?

[FireMasterK]

will you plan to release a separated docker image with that build?

There could be an image with all build features.

[paolafrancesca]

@FireMasterK fixed, pleae let me know if anything else is missing