Update connect-kotlin code generation to run within Gradle build

[pkwarren]

There have been issues in the past w/ crosstests, Maven, and other build flows where the Makefile is used but is missing required code generation dependencies (you have to manually add the generate prerequisite - see #95 for an example).

Ideally code generation would work in connect-kotlin so any user could run ./gradlew build and get a successful build without having to find the appropriate make targets. It could configure this so the compileKotlin gradle task requires code generation first.

This should be pretty easy to automate (we might even want to look at the buf gradle plugin for this) and it would reduce maintenance long term.

https://github.com/bufbuild/protovalidate-java/pull/46 is an example of this migration for another project.