Add extension support to tonic reflection server

## Feature Request

### Crates

* `tonic-reflection`

### Motivation

`grpcurl` requires the reflection server to implement all reflection methods to resolve types, including `all_extension_numbers_of_type`. Since the server does not implement these two methods, grpcurl won't resolve the types.

### Proposal

Can we implement these two methods? This issue is quite important to me so I'm willing to contribute to it.

### Alternatives

Add a flag or crate feature to return an empty response.