Add optional default unimplemented stubs

Thought I should create an issue with corresponding PR I submitted.

## Feature Request

### Crates

tonic-build

### Motivation

Re-implement/undo https://github.com/hyperium/tonic/issues/221 changes as optional feature.

This allows referencing gRPC models without version control which only permit backwards-compatible changes. It allows for setups provided by other programming language gRPC service generators that implement RPC's by default and return UNIMPLEMENTED error code for any methods which aren't explicitly handled.

### Proposal

https://github.com/hyperium/tonic/pull/1344

### Alternatives

I did try seeing if I can use some macro magic outside the library, but it becomes quite a hack.