protoc-gen: Refactor optional field generation (KRPC-262)

[Jozott00]

Subsystem
Protobuf/protoc-gen

Problem Description
Currently, optional fields that lack a user-defined default value are generated as nullable properties, with the exception of sub-message fields.

This implementation differs from the standard protobuf approach, which handles all optional fields as non-nullable. This allows users to quickly access default values for non-set subfields. Furthermore, in the edition 2023+ version, where all fields are typically optional by default, managing nullable properties becomes cumbersome.

Solution
This PR aligns the implementation with official Protobuf implementations, ensuring that all fields (except one-of fields that lack a default value) are non-nullable. However, to maintain Kotlin’s idiomatic nullable handling, users can enable additional nullable getter generation, which produces

// in MyProto.ext.kt
val MyMessage.myFieldOrNull: FieldType?

Additionally, MyMessage.Builder.clear<Field>() functions are generated for all fields that are presence tracked. This enables users to clear the original set value in the copy block

val copy = message.copy {
  clearMyField()
}

[Jozott00]

Not yet implemented:

• Gradle settings to enable nullabe extension getters
• Documentation update

[Jozott00]

Not sure if this is actually needed, or if we could just use the PropertyKey