Provisioner: Support warnings on errors, and downgrade unused-missing-credentials to a warning

[josephjclark]

Two parts to this issue:

1. A means of returning warnings when posting to the provisioner API
2. Don't error/abort when failing to provision a credential that is not used

Warnings

The provisioner returns a { data, errors } object right now

But we want to enable warnings - message which suggest a problem with the payload but which are not critical.

There are two ways we can do this:

1. Return a warnings array, like { data, errors, warning } (I think I prefer this - easier to reason about)
2. Return a severity key on each error in the { data, errors }

Credential Errors

When posting a project to the provisioner, any credentials in the credentials array will be linked. If the credentials don't exist, an error is thrown.

This can be a problem when deploying a project on cloud (which may have many dev credentials attached) to another instance.

The ideal solution is that if a credential is in the credentials array but is NOT actively linked to any workflows, report a warning but carry on processing the request.

What about credentials actively used by the workflow but are not present in the app? Should we create/update the project anyway and let a user manually set up the credential? Just let it fail at runtime? I don't think this is unreasonable given the decoupled nature of credentials. So maybe in this case we complete the request and create the project, but still return an error (not a warning)

See also

#4465
OpenFn/kit#1268