[enhancement] store balances at block heights - implementation

[joel-u410]

Pushing this as draft, in order to show my proposed changes here. This cannot be merged until the corresponding feature is merged in the SDK: anoma/namada#3530. After that's done, I'll come back here and update this to use the upstream SDK and not my fork.

This implements #77.

Example of the balance_changes table:

And corresponding balances view (which maintains the previous behavior of the balances table):

After a transaction occurs which updates the balance (note the id changes in the returned row in the balances view to reflect which balance is now the latest one):

[joel-u410]

Good news -- anoma/namada#3530 was merged, so I suppose after the next namada SDK release, this can be updated / rebased and will no longer require a forked version.