Support for deparsing of ArrayCoerceExpr node in contrib/postgres_fdw

Support for deparsing of ArrayCoerceExpr node in contrib/postgres_fdw

When using a prepared statement to select data from a PostgreSQL foreign
table (postgres_fdw) with the "field = ANY($1)" expression, the operation
is not pushed down when an implicit type case is applied, and a generic plan
is used.  This commit resolves the issue by supporting the push-down of
ArrayCoerceExpr, which is used in this case.  The support is quite
straightforward and similar to other nods, such as RelabelType.

Discussion: https://postgr.es/m/4f0cea802476d23c6e799512ffd17aff%40postgrespro.ru
Author: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
Reviewed-by: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>