Add grpc rust codegen + stubs, add rppal compile feature

for raspberry pi compilation.

This is because rppal won't compile on mac, so we
only want to bring in the dependency when actually compiling
for the raspberry pi, and so will manually need to enable the
dependency.

car-rs/Cargo.toml

@@ -7,11 +7,22 @@ edition = "2021"
 
 [dependencies]
 # https://github.com/golemparts/rppal
-rppal = "0.13.1"
+rppal = { version = "0.13.1", optional = true }
 tokio = { version = "1", features = ["full"] }
+prost = "0.11"
 
 # https://github.com/hyperium/tonic
 tonic = "0.8.0"
 
+# https://docs.rs/serialport/4.0.1/serialport/index.html
+serialport = "4.0.1"
+
 [build]
-target = "armv7-unknown-linux-gnueabihf"
+target = "armv7-unknown-linux-gnueabihf"
+
+[build-dependencies]
+tonic-build = "0.8.0"
+
+
+[features]
+rppal = ["dep:rppal"]