Add upgrade_at field to SoftwareUpgradeProposal

[DenisCarriere]

MintScan includes a timestamp for upgrade at

• uses timestamp of block of height
• consider that the block is in the future and does not have a timestamp yet until the block exists
• could do a @deriveFrom of block, however we do not know the "id" of the block until the future, perhaps we can change block.id to use number instead of id (hash), this would allow us to reference blocks in the past and whenever the blocks appear in the future, they would be linked.
• Need to consider being able to query SoftwareUpgradeProposal when the future block of upgrade_at does not yet exists

https://www.mintscan.io/cosmos/proposals/924