sql: add row count estimates to show tables

[awoods187]

It's important for developers to know the approximate table size in rows when optimizing query performance. Currently, you can't see table row size estimates in CockroachDB without running something like:

 select count (*) from rides;
  count
---------
    500
(1 row)

Time: 2.258ms

We should add estimated row counts to show tables and to the ERD proposed in #48418.

root@127.0.0.1:52983/movr> show tables;
  schema_name |         table_name         | type | rows
--------------+----------------------------+--------
  public      | promo_codes                | table | 1000
  public      | rides                      | table | 500
  public      | user_promo_codes           | table | 0
  public      | users                      | table | 50
  public      | vehicle_location_histories | table | 1000
  public      | vehicles                   | table | 15
(6 rows)

To avoid running the equivalent of select count (*) everytime we run SHOW TABLES we could use the last value collected from automatic statistics.

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