How to use system/different `protoc`

[james-callahan]

From a supply chain security point of view I don't want zig-protobuf to provide its own protoc: I'd like to either use my system one, or possibly build it from source. How can I tell zig-protobuf to use my system protoc?

[Arwalk]

Fair point, we might need to add options to RunProtoc. At first glance I'd say we need

• download (current behaviour, default)
• use system provided through path encmv
• use user provided protoc

[Arwalk]

done with #66