Allow setting protoc command via Meta

[duckbrain]

Fixes #3623

Adds a meta value protoc:cmd to override the command (and prefix args).

If the meta value is provided for both the service and the API, the API will be used an API-wide default and the Service definition will be used as a Service-level override. This seems to make more sense than appending them together like we do with includes.

I wasn't sure if it made more sense to have Meta("protoc:cmd") (no values) do nothing or restore the default protoc if used as an override in the Service definition, so I made it error. Also considered not allowing it on the service definition to avoid the situation.

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Tested this with a private project as well.

[raphael]

This looks great! I left a few suggestions - nothing major though.

[raphael]

Looks great! One tiny last modification: it would probably be good to have a header comment on the "DefaultProtoc" constant so that it is clearly documented in the dsl package online docs. Thank you!

[duckbrain]

Whoops, good catch.

Thanks for the great feedback, especially pointing me to the Meta option. I started out brainstorming how to handle multiple arguments on a CLI flag.

[raphael]

Thank you for the great contribution!