Add Gloas protobuf definitions with spec tests

[terencechain]

This PR introduces gloas protobuf support and also depends on recent changes in fastssz and go-bitfield, which reviewers should be aware of:

• fastssz previously lacked support for vectors of composite types and panic. Gloas introduced this with pending payments, and we fixed the related bug here: Fix composite types fastssz#17
• go-bitfield now supports PTC bitfields of size 2 for minimal config. That change is here: Add support for bit vector 2 go-bitfield#65