Enhancement: Add proposer to blocks

[purestaketdb]

Summary

The only way now to retrieve the proposer of a block is from Algod and in msgpack and encoded. It would be helpful to have it available from the Indexer.

Scope/Requirements

The addition of proposer to the data model, a migration script to add it to existing indexers, and adding it to the API.

Urgency/Relative Priority

It is helpful, but not urgent

[algoanne]

Looking at this now -

We can add the proposer to the block in the indexer. This will require a DB migration to add the column. We can do this. 2 pts.

Backfilling the data requires a more serious migration, requires getting all the historical blocks again. We are not sure whether users need the data backfilled.