Rename ambiguous Code attribute

[alpe]

Code -> CodeID

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #204 into master will not change coverage.
The diff coverage is 90.00%.

@@           Coverage Diff           @@
##           master     #204   +/-   ##
=======================================
  Coverage   71.71%   71.71%           
=======================================
  Files          27       27           
  Lines        2584     2584           
=======================================
  Hits         1853     1853           
  Misses        621      621           
  Partials      110      110           

Impacted Files	Coverage Δ
x/wasm/internal/keeper/test_common.go	86.89% <0.00%> (ø)
x/wasm/handler.go	50.00% <66.66%> (ø)
x/wasm/internal/keeper/handler_plugin.go	74.43% <100.00%> (ø)
x/wasm/internal/keeper/proposal_handler.go	69.76% <100.00%> (ø)
x/wasm/internal/types/msg.go	42.45% <100.00%> (ø)
x/wasm/internal/types/proposal.go	94.94% <100.00%> (ø)
x/wasm/internal/types/test_fixtures.go	91.51% <100.00%> (ø)

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[ethanfrey]

Good call.

Please add a migration guide file for the clients mentioning this

[alpe]

This PR not affect clients as the json name was code_id before and was not modified. See https://github.com/CosmWasm/wasmd/pull/204/files#diff-8c1459d12d1cdd42896fc113ef3b8335R170