Add support for http header routing

[gjreasoner]

Adds support for #1177, http header based routing pattern.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Changelog has been updated and is aligned with our changelog requirements
• Any necessary documentation is added, such as:
  • README.md
  • The docs/ directory
  • The docs repo

Fixes #1177

[wozniakjan]

thank you for contributing!

Before we move forward, I'd like to propose an alternative path for configuring the headers routing as part of the HTTPScaledObject CRD. Imho it would allow more flexibility if both, the header name as well as header value, could be configured together, similar to how rule matches are defined in HTTPRoutes in Gateway API

For example:

apiVersion: http.keda.sh/v1alpha1
kind: HTTPScaledObject
spec:
  hosts:
  - my.domain.com
  - my2.domain.com
  pathPrefixes:
  - /root
  - /new-feature
  headers:
  - name: x-header-test
    value: abc
  - name: x-header-test2
    value: def

xref #1177 (comment)