Move the proto files out of the Gradle project tree

[siburu]

Currently the proto files defined in Corda-IBC exist under proto/src/main/proto/..., but these paths are difficult to use and inappropriate, for other languages like Go or Rust.
To fix this, we should do:

• Rename the Gradle subproject proto to gentypes.
• Make a new directory proto and move the proto files out of gentypes/src/main/proto/... to the new directory.
• Fix gentypes/build.gradle to refer it to the proto files.
• Also fix go/... and rust/... to use the new paths.