`required` option for non-primitive types in protobuf

[kostysh]

The context:

• All non-primitive types in protobuf are optional. So if it is a field that refers to another type, it's optional.
• It's established in protobuf that compilation of a message field type is not defined as required or optional and is up to the compiler to determine how it is handled. Since protobuf 3 all fields that are primitives may be marked optional or required, but required syntax is now prohibited in proto3 as all primitives are assumed to be required unless marked optional.
• this way non-primitive type such as Date cannot be marked as mandatory

The task:

• investigate and figure out how to implement protobuf functionality that will allow us to mark non-primitive types as required
• all required types including non-primitive types must be properly reflected in the autogenerated typescript code