Cardano Updates

InstantaneousRewards are payments from either the treasury or the reserves
to a stake address. Since these are not associated with a stake pool the
`PoolId` field of the `Reward` table has now become NULLable. All entries
where the `Type` is `member` or `leader` should have a `poolId` entry while
for `treasury` or `reserves` should always have NULL for the `poolId` field.

Closes: https://github.com/input-output-hk/cardano-db-sync/issues/695

This value is the slot within the epoch after which the InstantaneousRewards
can be considered stable. The InstantaneousRewards can be extracted from
ledger state any time after this and before the end of the epoch.

Rewards from the ledger are either `RewardLeader` (rewards earned as the
pool owner) or `RewardMember` (earned as a delegatee).

There are however also instantaneous rewards that can come from either
the reserves or the treasury. This means we need a new type that includes
all four source.

Store information about Alonzo scripts in the database

Jira: https://jira.iohk.io/browse/CAD-3050
Jira: https://jira.iohk.io/browse/CAD-3161

db: Improve doc on Reward table and clarify when they are spendable