Is your feature request related to a problem? Please describe.
Currently, we don't have any validations on the ExternalSecretDataFromRemoteRef, but while reading the source code I noticed there are several pre-conditions we need to meet:

1. We should set either Find, Exact, or SourceRef & SourceRef.GeneratorRef.
2. We should set SourceRef.GeneratorRef if we want to use SourceRef.

Describe the solution you'd like
Those conditions are not validated properly right now, so it would be better if we add validations here.