Allow IDEs to recognize generated code

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

This issue is a clone of apache/datafusion#3332.

Presently, tonic generates code into the target/ folder, which causes IDEs such as CLion to not be able to go to definition, or provide any auto-complete for this code, making it hard to work with.

Describe the solution you'd like

I'd like to move the generated code under the src/ folder, but .gitignore it.

Describe alternatives you've considered

• Leave as is
• Check in the generated code