gov: add prune wasm codes proposal

[alpe]

In order to release storage space for unused contracts a prune proposal makes sense.
When executed wasm codes without an instantiation would be removed from CodeInfo and wasmvm.
The proposal may include a relative height (or fix one in the past?) as a threshold to not delete new uploads.

• Should exclude pinned code ids

Follow up on:
#840 (comment)

[webmaster128]

You can shedule this for 0.31 and start the work if you want to. The cosmwasm/wasmvm code will be ready as part of CosmWasm 1.2.

[pinosu]

#1455 was closed after doing some benchmarks because of poor performance. Most likely a different approach is needed e.g. a migration script. Since this task is going to take more time than expected, it is descoped from v0.41 milestone.