[feature]: count unconfirmed mints at SQL level in `ListAssets` call

[guggero]

We should address the following TODO:

taproot-assets/rpcserver.go

Line 1166 in 4f89af9

// TODO(guggero): Do this on the SQL level once we add pagination to the

Especially when (or before) implementing #1707.

We count the number of unconfirmed asset mints (and also do the filtering of unconfirmed mints based on the include_unconfirmed_mints request parameter) in the Golang code instead of doing it in SQL.
Which means the numbers won't be correct anymore once we implement pagination, as the unconfirmed_mints response field would only contain the number of unconfirmed mints of the current page, not the global number.