`SecretKey` should be enough to verify note's ownership

[ZER0]

Summary

We created the ViewKey as a way to delegate the ownership check, however we don't always do that: for example, in the current implementation of the wallet (CLI and web) we're verifying the note locally. In order to do so, we need to create a ViewKey even if we have access to a SecretKey, that should be enough to verify the note's ownership.

Possible solution design or implementation

As the ViewKey has, the SecretKey should implement the owns method as well.

Additional context

The hope is to reduce not only the code involved, but at least a bit of the operations too.