Support for GiST in get_equal_strategy_number()

Support for GiST in get_equal_strategy_number()

A WITHOUT OVERLAPS primary key or unique constraint is accepted as a
REPLICA IDENTITY, since it guarantees uniqueness.  But subscribers
applying logical decoding messages would fail because there was not
support for looking up the equals operator for a gist index.  This
fixes that: For GiST indexes we can use the stratnum GiST support
function.

Reviewed-by: Paul Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com