docs(samples): add vault-provider.yaml showing generic loadFromSecret approach

[vdice]

I wanted to add the loadFromSecret generic runtime config approach to a sample somewhere for improved visibility.

Not sure if this is the best approach. Could also create a new sample to put this in. Thoughts?

[endocrimes]

A new sample would be preferred - because it can also show how to use it.

That being said - I'm not sure if we're going to keep this long term? - it was kinda an escape hatch during early development that I think we've outlived the usefulness of.

[calebschoepp]

I'm in support of removing LoadFromSecret support.

[vdice]

I'm in support of removing LoadFromSecret support.

The loadFromSecret functionality is currently useful for providing Vault and/or Azure Key Vault application variable provider config to SpinApps, since there is no typed support in the Spin Operator, yet. Perhaps consider removing after #272 lands -- assuming it represents the last bit of typed runtime config to be added for parity with Spin?

Also note the integration between loadFromSecret and the kube plugin.

[vdice]

As for this example, happy to close if indeed we'd like not to advertise this functionality. I just found it useful for my use case.

If we'd like to keep this sample manifest, I can also add a corresponding sample app to apps if that would be preferred.