Membership Services refactoring: separate and break down the code to more files

[JonathanLevi]

Description

While the good ol' Membership Services CA package implementation has some logical separation to sub-modules/structs, we can break down the implementation to separate files (e.g., move Google's Protocol Buffers language-neutral data serialization/API to separate/dedicated files, refactor TCAA and TCAP out of tca.go, ECAA and ECAP from eca.go and ACAP from aca.go)

Describe How to Reproduce

N/A. Code restructuring, not a bug.

[christo4ferris]

+1 suggest the refactor be gradual as possible and that unit tests are added where appropriate.

[JonathanLevi]

OK, thanks. Will break it down to separate PRs too, as suggested. Will begin with the easy/straight forward ones (protobuf & co.)