Better headers handling configuration

[marcinczeczko]

Version used

• knot.x: any

Context of the issue/feature

There is a general problem in how to configure what headers to allow/deny either on request or response. There is a need to unify the approach. The proposed solution is have config section as below instead of allowRequestHeaders

    requestHeadersPolicy = [ # or responseHeadersPolicy
      {
        allow: ".*"
      },
      {
        deny: Host
      },
      {
         overwrite.X-Header = my-new-value # will override header if exists
      },
      {
         add.X-Header = my-new-value # will send a header with a given value, no matter if it exists
      }
    ]

The idea is that the user sets the policy of how headers are to be handled using allow or deny directives, so he/she can allow everything and then deny particular headers, or opposite way, allow everything and blacklist just specific headers.
Additionally, a policy could be used to overwrite existing or add new headers to the requests.

Bug reproducer

N/A

Steps to reproduce

N/A

Appendix

[marcinczeczko]

Contribution in form of ideas, comments or PR are welcome.

[dragarthPl]

I have a question.

Does the configuration have to look like above or can it look like this:

requestHeadersPolicy { # or responseHeadersPolicy
    allow = [".*"]
    deny = [Host]
    overwrite.X-Header = my-new-value # will override header if exists
    add.X-Header = my-new-value # will send a header with a given value, no matter if it exists
 }

If answer is yes, so how we should play with multi value headers ?

Maybe we could do it with:
requestHeadersPolicy { # or responseHeadersPolicy
allow = [".*"]
deny = [Host]
overwrite.X-Header = my-new-value # will override header if exists
add.X-Header = [my-new-value] # will send a header with a given value, no matter if it exists
}

First we overwriting specific header, and then we add rest off all values for that header.

For reflection: will there be header values ​​with phrases that may interfere with the configuration phrase?