[feat] add TWAP last updated slot to DB and indexing

[R-K-H]

A feature request we had was to determine the last time the TWAP was cranked or evaluated and what that was evaluated at:

We have some of this setup in the DB now, but it does seem we're missing a few fields being put into the DB like last_observation and last_price.

Once this is configured we want to set it up so we can leverage the SDK to subscribe and the front end display this value somewhere within the trading page.