Introduce new interfaces for extendability

[alpe]

Refactoring towards a more modular design.

• New interfaces in x/wasm/types/exported_keepers.go
• PermissionedKeeper decorates the keeper with authorization policies and provides public methods to provision or interact with a contract
• Minor bugfix for registering proposal types with amino
• Emit multiple events for pin/unpin instead of list attribute

[codecov]

Codecov Report

Merging #471 (48df71f) into master (388469c) will increase coverage by 0.04%.
The diff coverage is 85.29%.

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
+ Coverage   56.76%   56.81%   +0.04%     
==========================================
  Files          40       41       +1     
  Lines        4385     4390       +5     
==========================================
+ Hits         2489     2494       +5     
- Misses       1685     1686       +1     
+ Partials      211      210       -1     

Impacted Files	Coverage Δ
x/wasm/handler.go	74.28% <ø> (ø)
x/wasm/keeper/msg_server.go	0.00% <ø> (ø)
x/wasm/keeper/options.go	73.33% <ø> (ø)
x/wasm/keeper/query_plugins.go	76.51% <ø> (ø)
x/wasm/types/keys.go	38.23% <ø> (ø)
x/wasm/ibc.go	59.37% <50.00%> (ø)
x/wasm/keeper/genesis.go	88.05% <50.00%> (+0.18%)	⬆️
x/wasm/keeper/keeper.go	86.00% <60.00%> (-0.12%)	⬇️
x/wasm/keeper/querier.go	50.81% <75.00%> (ø)
x/wasm/keeper/legacy_querier.go	68.75% <83.33%> (ø)
... and 8 more

[ethanfrey]

Looks good. A few minor comments, but does make extensibility points much clearer.