feat(CRDs-schema): add global and enabled placeholders for subchart/dependency

[ArlonAntonius]

What does this PR do?

Fixes #1311
Superseeds #1334
TL:DR Allows the usage of the traefik-crds chart as a subchart/dependency.

Motivation

To use the traefik-crds Helm Chart as a dependency in your projects, the validation needs to be slightly looser to support the usage of the chart. Helm passes specific values by default (global in this case), causing the Schema Validation to give an error.

In line with other Helm Charts (like cert-manager), it would be beneficial to allow users to set:

• global to fix the error that will always be caused by Helm passing the global values
• enabled to provide users with the default (recommended) flag for conditions on Helm Charts

The one thing that I did, that is more opinionated and would like the opinion of Traefik's maintainer on is the usage of additionalProperties on the global object. Cert-manager specifically provides a set of variables, defeating part of the feature's purpose (in my opinion).
https://github.com/cert-manager/cert-manager/blob/23d3aea079ce88cec6f9394f4a4027720f8d7c2a/deploy/charts/cert-manager/values.schema.json#L686

More

• Yes, I updated the tests accordingly
• Yes, I updated the schema accordingly
• Yes, I ran make test and all the tests passed

[ArlonAntonius]

@mloiseleur Especially interested to hear your take on this as I know you are involved in #1334 and have not provided a specific opinion on #1331 yet

[mloiseleur]

@ArlonAntonius I guess you mean on #1311 . We can add required properties from some use cases and keep the benefits of the schema.

FYI, the schema is generated from values:

traefik-helm-chart/Makefile

Lines 30 to 35 in 6350cb1

	# Requires to install schema generation plugin beforehand
	# $ helm plugin install https://github.com/losisin/helm-values-schema-json.git
	schema:
	cd traefik && helm schema
	cd traefik-crds && helm schema

Would you please update your PR accordingly, so the schema will be kept ?

[ArlonAntonius]

@mloiseleur Made the required changes, let me know if this aligns correctly with Traefik's coding and documentation standards 😉
If it's easier to discuss something in real-time, feel free to let me know.

[mloiseleur]

@ArlonAntonius See my suggestion for improved documentation. Otherwise, LGTM.

[ArlonAntonius]

@mloiseleur Included your change 🚀 Any chance we can include this in the release you have planned in #1351 ?

[mloiseleur]

@ArlonAntonius You need to rebase this PR (and/or allow edit from maintainers)