feat: introduce konghq.com/headers-separator annotation and make parsing more robust

[programmer04]

What this PR does / why we need it:

This PR resolves real-world problems reported by the user that konghq.com/headers.* doesn't allow to specify headers that contain , which is permitted in header values, but can't be used because KIC uses it as a separator.

HTTP specification is very permissive in terms of what is allowed as header value
https://datatracker.ietf.org/doc/html/rfc9110#name-field-values

Field values are usually constrained to the range of US-ASCII characters [USASCII].

see also Cloudflare docs.

Hence coming up with an escaping pattern for commas is hard and won't be easy to use/interpret by a user. So this PR introduces an additional annotation konghq.com/headers-separator that allows specifying a different separator than the default ,.

Moreover, this PR makes parsing header values from konghq.com/headers.* more robust - now it discards leading and trailing whitespace characters so the below annotations are equivalent

konghq.com/headers.foo: foo,bar,baz
konghq.com/headers.foo: foo, bar   , baz

this is a safe change, because according to https://datatracker.ietf.org/doc/html/rfc9110#section-5.5-3

A field value does not include leading or trailing whitespace. When a specific version of HTTP allows such whitespace to appear in a message, a field parsing implementation MUST exclude such whitespace before evaluating the field value.

that will make users' lives easier.

Furthermore, a test case for handling a header with an empty value has been added because it's a valid use case, see https://datatracker.ietf.org/doc/html/rfc9110#section-12.5.3-12

An Accept-Encoding header field with a field value that is empty implies that the user agent does not want any content coding in response.

Which issue this PR fixes:

Closes #5427

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR
• docs has been updated too - kic: document newly added konghq.com/headers-separator annotation docs.konghq.com#7351

[czeslavo]

👍