pb: Add presence check support

[Jozott00]

Subsystem
Protobuf

Problem Description
Even if a field in a message is optional, we can't make it nullable. This is because fields may either have a default value, or we want to be able to access a default value in a nested field such that the parent object must not be null.

However, users must be able to distinguish between a field that was manually set with the default value and one that was not actively set.

Solution
This PR solves this problem by adding a presence API to each message that contains fields with presence tracking.

val myMessage = Message { myField = "value" }
myMessage.presence.hasField

presence is an extension getter on the generated message interface that provides a hasXYZ getter for all fields with tracked/explicit presence.
This intermediate presence object is employed to ensure concise code completion and minimize the number of suggestions provided by the IDE.

The only field type that lacks a hasXYZ getter while its presence is tracked is the oneof field. Since oneof fields are nullable, they are null if they are not set.

[Mr3zee]

@Jozott00 great and quick job!
I won't have time to look into it closely today, but I noticed two things after a quick glance:

• Presence class that is generated is in __rpc_internal package, which is fine for it's implementation, but I think we should expose to the user an interface with hasField properties in .ext.kt and keep the impl for it in __rpc_internal, so that the navigation doesn't bring you to internal when possible
• No generated docs for .presence

[Jozott00]

@Mr3zee

1. Yes, that is probably better. Any suggestions on how to name the interface? Currently it is an inner class which makes it easy. When moving it to the .ext.kt should I name the interface <OuterClass><InnerClass>Presence? (assuming it's the interface for a nested message)
2. Good point, I will add documentation

[Mr3zee]

@Jozott00 good question, I don't know 😄 Extension classes would be great here

[Mr3zee]

what about

interface OuterClassPresence {
    val hasField: Boolean
	
    interface InnerClass {
        val hasInnerMessageField: Boolean 
    }
}

[Mr3zee]

Also, please update the conformance regeneration