Update module spec towards service oriented architecture

[robert-zaremba]

Summary

In past year we checked few approach for designing modules and transaction messages. ADR-31 introduced Protobuf Service oriented architecture. Recently we simplified service request messages and types and update the documentation.
Our module specs are still oriented towards messages rather then services. Core modules act as a template and a guideline for all other modules. Hence we should update the documentation of the core modules towards the architecture we are implementing.

Proposal

In authz module spec, we drafted how the the service oriented spec can look like and decided to roll it back and have a separate issue to update the SPEC-SPEC document together with a module spec to present a

• Update the SPEC-SPEC
• Update x/authz/spec files

Once done, we should create follow-up tasks to migrate other module specs.

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[robert-zaremba]

Plan:

• Propose updated spec
• make a post
• communicate

[julienrbrt]

spec-spec has been simplified to not require that (https://github.com/cosmos/cosmos-sdk/blob/main/docs/spec/SPEC_MODULE.md)
we've simplified the docs of other modules.

the way to build module is to use core, which will be reflected in: #21429