generate classes with non-null Message types with custom options / extensions

[Dogacel]

Refers to

• #242

Overview

Allow not nullable Message types to be generated by using custom protobuf options or extensions.

• Added tests to verify stuff are working fine.

How

• Add a new option called required_field_extension_number.
• Update CodeGenerator to allow nullable MESSAGE type and set the default value to be the constructor invocation (default instance)

Risks

• Extension number clashing, I find it unlikely but could be nice to double-check.
• Circular dependency, I left this one to the user. Circular paths with not nullable messages lead to infinite recursion.