Allow generating to return a string instead of writing to out_dir

## Feature Request

Right now, the generation is happening to `out_dir` and creating a file. I would like it if I can get a string out of the generation and manipulate it.

### Crates

`tonic-build`

### Motivation

This is because we want to have much better control over when things are generated, commit the generated files to provide an SDK framework, and would like to add more testing to make sure protobuf files are always generated.

### Proposal

### Alternatives