Add functions to generate VRF Proof to PrivValidator-derived classes

[torao]

To generate a VRF Proof, it needs to implement an additional function in a subclass derived from the PrivValidator interface that holds a private key to generate the signature.

The target subclasses derived from the PrivValidator are:

1. FilePV has a key pair mapped to a local file and uses it to generate signatures.
2. MockPV generates and keeps a key pair on the memory and uses it to generate signatures for tests.
3. SignerRemote provides RPC functions to delegate signature to PrivValidator of the remote process.
4. SignerValidatorEndpoint uses Mutex for each SignerRemote method call to acquire a monitor and synchronize it.

This issue is to add the GenerateVRFProof() function to these classes.

[torao]

The unit-test has been finished. I'll create PR after #4 will be merged.

[egonspace]

Why is GenerateVRFProof() added to the subclass, not to PrivValidator itself?
If you add it to the subclass, ConsensusState has the subclass as field instead of PrivValidator, is it right?

[torao]

PR #15 has been merged.