[YSQL] Support multicolumn GIN indexes

[jaki]

Jira Link: DB-892

[jasonyb]

Currently, amroutine->amcanmulticol = false. This means that gin indexes can only index a single column, not multiple columns. There's also amroutine->amcaninclude = false, but that is a limitation with upstream postgres as well.

Note that the btree_gin extension is most useful with multicolumn because it allows gin indexed columns to be alongside regular indexed columns.

keywords: multi-column multi column btree btreegin btree_gin btree-gin extension