feat(x/callback): Add module skeleton

Implement the skeleton of the module. It should contain only the basics of the module, no implementation of business logic, in order to make PR reviewing less noisy.

Expected components

.
├── client
│   └── cli
│       ├── query.go <= only the QueryCmd() function which panics
├── genesis.go <= only the default Export/Import/Validate with panicking implementation.
├── keeper
│   ├── keeper.go <= only the keeper type and the constructor.
│   ├── params.go <= only the params validation function which panic
│   ├── query.go <= only the empty query server implementer + constructor.
├── module.go <= app module + app module basic.
└── types
    ├── codec.go
    ├── errors.go
    ├── expected_keepers.go BLANK
    ├── genesis.go <= DefaultGenesis function
    ├── genesis.pb.go
    ├── keys.go <= the module constants (ModuleName/StoreKey...)
    ├── params.go <= panicking params validation function.
    ├── params.pb.go 
    ├── query.pb.go
    ├── query.pb.gw.go
    ├── tx.pb.go
    └── types.go

archway-network / archway

feat(x/callback): Add module skeleton #483