Preserve Flow<...> in returnType and add returnsFlow flag on RpcCallable

[MiletaA]

Subsystem
Compiler Plugin, Descriptor Core

Problem Description
Descriptors for RPC methods that return Flow<T> were exposing returnType.kType == T instead of the declared Flow<T>.
It is issue #523

Solution
Stop unwrapping Flow<T> to T in RpcStubGenerator, so the generated descriptor now keeps returnType.kType == Flow<T>.
Add val returnsFlow: Boolean to RpcCallable and implement it in RpcCallableDefault; the compiler plugin sets this flag whenever the callable’s return type is a Flow<*>.

[Mr3zee]

@MiletaA thank you for the contribution!
This is good contribution in terms how you handled compiler code and tests. However I want to point out some problems with it:

• It is a breaking behaviour for kRPC and tests will fail
• Flow as a type doesn't have a lot of sense from the API perspective, it's better to use typed information. You can check out how we did it in a branch already:

  kotlinx-rpc/core/src/commonMain/kotlin/kotlinx/rpc/descriptor/RpcServiceDescriptor.kt

  Line 72 in cda97dc

  public sealed interface RpcInvokator<@Rpc Service : Any> {

[Mr3zee]

If you want, you can backport the feature from the grpc branch into main in this PR, and I'll guide you through

[MiletaA]

@Mr3zee No problem, thank you for the feedback, I will check the problems you have pointed out. Hopefully, I have backported the correct feature, If so, what is the next step?

[Mr3zee]

@MiletaA no, this is unrelated part of another feature. I was pointing out to the particular code part, not a commit. To backport it you'll need to extract the change in the descriptor from others in the branch.