Improve feedback when non-suspending function included in RpcService

[chris-hatton]

By their nature; all member-functions of RpcService implementations should be suspending.

During development, it may be a common mistake to omit suspend, in which case the developer-user is faced with a potentially confusing error at compile time:

e: org.jetbrains.kotlin.backend.common.BackendException: Backend Internal error: Exception during IR lowering
File being compiled: /Users/user/Projects/project/shared/src/commonMain/kotlin/com/project/shared/domain/service/SomeService.kt
The root cause java.lang.RuntimeException was thrown at: org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:47)
...
<long stack trace>

Describe the solution you'd like
Compiler plugin or KSP part of kotlinx.rpc should detect the presence of non suspend functions and throw a more meaningful error e.g:

The function 'getUser' in com.company.project.service.UserService is not suspending, only suspending functions are allowed in RPC service implementations.

[Mr3zee]

Hey, @chris-hatton !
Thanks for suggestion! We are planning to introduce more user friendly checkers, including this one as one for the most needed