Refactor validator server naming and modules

[sergerad]

Stacked PR.

Creates clearer separation between validator server scaffolding (ValidatorServer) and api impl / business logic (Validator).

• Validator renamed to ValidatorServer (has ValidatorServer::serve()) and ValidatorServer renamed to Validator
• validator module added to validator server module
• error renamed to ValidatorError

From

server/
    mod.rs
    sign_block.rs
    validate_block.rs
    ...

To

server/
    mod.rs
    validator/
        *.rs

[Mirko-von-Leipzig]

I agree with the concept, but I dislike the raw validator nesting in there. Maybe we can move to a common pattern across the crates of:

server/
  mod.rs
  xxx_service/
    mod.rs
    method_a
    method_b
  yyy_service/
    mod.rs
    method_y
    method_z
  other_thing

so in this case I guess the primary objection I have is that Validator should be ValidatorService or something indicating Grpc in the validator_service or similar?