Generate `MessageDef` implementations for Proto messages

[Oleg-Melnik]

This PR includes the following tasks done:

• Implementations of the MessageDef interface are generated for Proto messages.
• Objects are generated instead of classes for all the implementations of MessageField, MessageOneof, and MessageDef interfaces.
• A mini-framework for code generation is implemented, providing a flexible way to change the contents of the generated files.
• A message builder extension is generated returning the corresponding MessageDef implementation.
• The project is migrated to the latest version of ProtoData 0.60.2.

The implementation of the MessageDef interface for some Proto message looks like the following:

public object IpAddressDef : MessageDef<IpAddress> {
    public val ipv4: IpAddressIpv4Field = IpAddressIpv4Field

    public val ipv6: IpAddressIpv6Field = IpAddressIpv6Field

    public val `value`: IpAddressValueOneof = IpAddressValueOneof

    public override val fields: Collection<MessageField<IpAddress, out Any>> = listOf(
            ipv4,
            ipv6)

    public override val oneofs: Collection<MessageOneof<IpAddress>> = listOf(
            value)
}