speedup COPY TO for partitioned table.

hi.

COPY (select_query) generally slower than
table_beginscan.. table_scan_getnextslot ..table_endscan,
especially for partitioned tables.
so in the function DoCopyTo
trying to use table_beginscan.. table_scan_getnextslot ..table_endscan
for COPY TO when source table is a partitioned table.

----setup-----
CREATE TABLE t3 (a INT, b int ) PARTITION BY RANGE (a);
create table t3_1 partition of t3 for values from (1) to (11);
create table t3_2 partition of t3 for values from (11) to (15);
insert into t3 select g from generate_series(1, 3) g;
insert into t3 select g from generate_series(11, 11) g;

so now you can do:
copy t3 to stdout;

in the master, you will get:
ERROR: cannot copy from partitioned table "t3"
HINT: Try the COPY (SELECT ...) TO variant.

attached copy_par_regress_test.sql is a simple benchmark sql file,
a partitioned table with 10 partitions, 2 levels of indirection.
The simple benchmark shows around 7.7% improvement in my local environment.

local environment:
PostgreSQL 18devel_debug_build_382092a0cd on x86_64-linux, compiled by
gcc-14.1.0, 64-bit