Add support for alternate credentials secret field

[deitch]

As discussed with @prashanth26 and @AxiomSamarth in this comment, we should support having the alternate credentials field.

The PR that added support to MCM and back-filled it into existing in-tree providers is here.

Sample validation would be:

		if "" == string(secret.Data[api.APIKey]) && "" == string(secret.Data[api.AlternateAPIKey]) {
			allErrs = append(allErrs, field.Required(fldPath.Child(api.APIKey), fmt.Sprintf("Required Equinix Metal API Key one of '%s' or '%s'", api.APIKey, api.AlternateAPIKey)))
		}