[SPARK-3537][SQL] Refines in-memory columnar table statistics

This PR refines in-memory columnar table statistics:

1. adds 2 more statistics for in-memory table columns: `count` and `sizeInBytes`
1. adds filter pushdown support for `IS NULL` and `IS NOT NULL`.
1. caches and propagates statistics in `InMemoryRelation` once the underlying cached RDD is materialized.

   Statistics are collected to driver side with an accumulator.
This can potentially slow down caching process of string columns, because the `.getBytes("utf-8")` call within `actualSize` traverses the whole string.