feat: Add Kotlin (JVM) support

[Zetten]

Provide rules to build Kotlin proto and grpc libraries. Currently upstream Protobuf and gRPC support only JVM-flavour Kotlin.

Fixes #139.

A few notes on implementation:

• Mostly simple, largely copied from the Java implementation.
• An updated routeguide example was adapted from https://github.com/grpc/grpc-kotlin - with renamed packages to avoid class collision with the Java example.

Some points which may want attention before merging:

• WORKSPACE ordering had to be rearranged a bit - grpc_extra_deps imports its own (older) rules_kotlin which got messy.
• Not sure how best to document the optional java_<type>_target parameter on the macro - the Kotlin rules depend on the compiled Java library for the same proto, but users may want to build them separately for some reason.
• Naming would ideally be kt_jvm_* under the kotlin directory, but this doesn't seem simple to do without heavily modifying the rulegen templates. Probably not a huge problem.