Support loading Helm values from different source types

[hiddeco]

Fixes #1809

valuesFrom supports different types of sources: configMapKeyRef,
secretKeyRef and externalSourceRef. Where the latter can be used to
retrieve a values file from an URL by setting the
externalSourceRef.URL. The array of sources is merged in specified
order, with the .spec.values getting merged last.

For backwards compatiblity the old valuesFromSecrets are transformed
into secretKeyRefs and lined up in front of any specified
valuesFrom sources.

On all source types the <sourceRef>.Optional can be set to true to
allow the release of a Helm chart to continue if the values file could
not be found.

[squaremo]

Reading values from a file is a bit odd -- the path is given in the HelmRelease, but refers to a file in the filesystem of the operator. This is a trapeze act too far, in my opinion (see #1468 for previous discussion. I see I used the trapeze metaphor there as well; I need to find another).

[squaremo]

I'm not against fetching values from URLs -- though it's not very "GitOps", I think on balance it's reasonable to give people some flexibility on where the values come from. (Or to put it another way: it's not something I'd do myself, but who am I to judge ...).

[hiddeco]

I do agree with you that reading values from a file is a bit odd, but at the same time I think it is also oddly specific to create a valueFileURLs array in the CRD.

[squaremo]

12/10 would review again

There's a couple of mechanical fixes I spotted; and, a suggestion about the CRD schema for you to consider.

[hiddeco]

@squaremo I think this is now to your liking.

[squaremo]

Ah! I didn't know the directive could go here, but this works.