Get creation date on the execution

[antho1404]

Right now the execution is "timeless", we have no way to know when the execution happened. We should have a "created_at" time set when the validator puts it in the block to be able to sort executions by time when we list them. Also, this will give us the possibility to actually filter by date of execution which would be really nice to debug.

Note: Creation date was initially present in the execution but got deleted for simplification purposes.

[NicolasMahe]

We could add the same feature to other resources as well.
It will actually solve issue #1590

[krhubert]

I would rather put the BlockId when execution was added, otheriwse we might have it :)

[NicolasMahe]

Let's use ctx.BlockHeight() or ctx.BlockTime() using sdk.Context in handlers.

[antho1404]

Block height is enough, we can get the time based on the block height but not the other way around