tonic-build: Allow setting skip_protoc_run on the prost Builder

[pruthvikar]

Motivation

prost-build has a configuration option called skip_protoc_run that prevents running protoc. This option is not exposed in tonic-build. The option can be used to run protoc externally or to use an alternative like protox.

Solution

This PR adds the option to set skip_protoc_run which is passed on to the prost-build configuration.

[LucioFranco]

#1326 will fix CI for you, so once that is merged should be able to rebase and get green ci

[LucioFranco]

Its been merged you can reabase and CI should work.