Partial decoding

### Things to check first

- [x] I have searched the existing issues and didn't find my feature already requested there

### Feature description

Is it possible to decode only the desired keys and not the whole blob? Something like `cbor2.load_keys(...)` or specify some hook in the `load` method to keep only desired keys.

### Use case

I think it would be useful to decode only part of data to save some RAM in different use cases. For example, I can read only the header and then forward the blob to another service or collect blobs as chunks and read only metadata from each blob.