This is a follow-up to #8938, a proposal to implement two new functions:

• crdb_encode_pkey(table_or_index_name, expr...), returning bytea, with table_or_index_name a string or identifier (string would be more easy and would create an opportunity to play with the virtual schema). This would compute and return the KV-level key encoding for the primary key of the specified table/index using the values given in the other arguments.
• crdb_current_range_bounds(pkey), where pkey is a bytea, returning a tuple of two bytea, with the two values are the current known range bounds for the range containing pkey.

The two functions in combinations can then be used to estimate "distance" between two rows in a table, in the sense of whether they belong to the same range or not, and can be used as a follow-up to #8938 to check that a split requested with "ALTER TABLE ... SPLIT" was actually processed.

The function names are just proposals.

cc @mjibson