sql: support inverted indexes with more than one inverted column

[jordanlewis]

Currently, it's not possible to create a multi-column inverted index where more than one column is inverted:

[email protected]:26257/defaultdb> create table a (j1 json, j2 json);
CREATE TABLE

[email protected]:26257/defaultdb> create index on a using gin(j1, j2);
ERROR: column j1 of type jsonb is only allowed as the last column in an inverted index
SQLSTATE: 0A000
HINT: see the documentation for more information about inverted indexes: https://www.cockroachlabs.com/docs/v22.2/inverted-indexes.html

However, this is possible in Postgres:

jordan=> create table a (j1 jsonb, j2 jsonb);
CREATE TABLE
jordan=> create index on a using gin(j1, j2);
CREATE INDEX

Having more than one inverted column would improve the selectivity of the index in some cases, such as a situation with two trigram indexes on two separate text columns in the same table.

For example, see this comment #41285 (comment)

Jira issue: CRDB-21271