bugfix(gateway-apis): Specific filters without BackendRefs

[larivierec]

Description of the change

Moves some parts to validation ensuring that a route will not be created with incorrect parameters. This also allows us to set a filter without a backend ref in case it goes directly to the Gateway for reprocessing

Fixed

Using RequestRedirect is not allowed with BackendRefs. I added a check to validate this and fail if it's present.

Additional information

Checklist

• [X] Title of the PR conforms to the Conventional Commits standard.
• [X] Scope of the of the PR title contains the chart name.
• [X] Chart version in Chart.yaml has been bumped according to Semantic Versioning.
• [X] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [X] Variables have been documented in the values.yaml file.

[larivierec]

hi @bjw-s, when you get time, no stress. i noticed this issue pop up during some of my testing.

it can be merged if ever you decide to cut a new release note: squashed for cleanliness

[larivierec]

@bjw-s i realized that backendRef will still be rendered even if I don't put anything.

This change is basically on validation, in order to properly make this work; I'd have to remove the default field in values.yaml