Allow to select docker registry credentials

[andresmgot]

Description of the change

Allow to add docker registry credentials in the spec of an AppRepository. This is still not ready to be reviewed but I'm sending a draft to validate the UX first:

Applicable issues

• fixes Support docker registry secrets for AppRepositories #1522

Additional information

still needs to be properly tested

[absoludity]

Thanks Andres! Yes, the UX looks exactly as I'd expect (ie. can select existing kubernetes.io/dockerconfigjson secrets as well as create new ones).

I'd move the "Server" field up to be the first piece of info after the secret name in the form.

Separate to this PR, we may want to use some validation on the server too (a few ways that could go).

[absoludity]

Separate to this PR, we may want to use some validation on the server too (a few ways that could go).

We will need to ensure that this is disabled (but probably good to be visible) when not using helm3 (already in the dashboard config). Also need to ensure it's disabled or explicitly not available for global app repos (in fact, if it's not visible for global repos, you won't need a feature flag, as it'll only display on per-app repos).

[andresmgot]

👍 thanks for the comments! I will try to split these changes so they are easier to review

[andresmgot]

closing this in favor of #1684 and #1685