feat(CRDs): add gatewayAPI experimental channel option

[discostur]

What does this PR do?

• adds the ability to install gatewayAPIExperimentalChannel
• you can switch whether you want to install standard or experimental channel
• i added the gateway api release v1.2.1 (like standard channel); might be updated soon because 1.3.0 should be supported with traefik 3.5.0

Motivation

• fix [CRD] Support Gateway API Experimental CRDs #1308
• fix TCPRoute CRD missing on clean install #1253

[mloiseleur]

Thanks @discostur for this PR.
We have a consistency check between CRDs of regular Chart and CRDs of this additional Chart.

You'll need to update this CI check accordingly, and also ensure version are consistent (ie: versions are the same between experimental and standard)

Would you please also update to Gateway API v1.3.0 ?

[discostur]

@mloiseleur version is updated to v1.3.0; i'm not sure if i understand it right but i updates the consistency script to support the check between standard and experimental CRDs (in both charts, traefik and traefik-crd).

[bpsoraggi]

Could you please add some tests? Otherwise looks mostly good to me

[discostur]

@bpsoraggi

• thanks for your suggestions, all accepted
• tests added
• fixed the if condition that experimental wins over standard if both are active

[bpsoraggi]

Thank you for the changes! Just minor fixes now

[discostur]

@bpsoraggi fixed

[bpsoraggi]

LGTM! Thank you for your contribution

[mloiseleur]

Overall, the PR LGTM. I have concerns about the values.

Unless I missed something, I understand that with this implementation, user can enable both standard and experimental channel. When there is a breaking change in experimental channel, it may lead to severe consequences.

@discostur Would you please update the PR to avoid that ? I see two options:

1. Turn gatewayAPI setttings into a list of two options (standard and experimental)
2. Add requirements (like those on traefik config) on values to fail gracefully before it's applied when user enable both channels