SignalSchema.to_partial refactoring

[dreadatour]

In https://github.com/iterative/datachain/pull/927 we have new to_partial method introduced in SignalSchema class. It uses signal schema serialization -> deserialization to create partial schema, and it looks like not perfect solution. Let's revisit this code again and implement proper solution without serialization.

Also check this thread, same issue with using serialized schema, it may be possible to avoid it and refactor this code as well.