storage: SHOW SIZES command

Customers ask for a SQL statements that gives them the cluster size utilized, and free space.
I understand the definition of "size" varies greatly depending on what we want the size of, so I wonder if we can work collaboratively and provide a shortcut (`SHOW SIZES;`) for this very common question we get from customers.

Ideally, we should provide the size of, roughly:
- the dataset, that is, non-replicated size of the data, as if I have to export it to CSV.
- the dataset as above, with revision history.
- the dataset, replicated.
- the physical space used by all SSTs.
- the physical space that is still available.

[slack](https://cockroachlabs.slack.com/archives/CHVV403F0/p1689868306287839)

Jira issue: CRDB-30172