[AQUMV]Enable answer query using Materialized View for external table.

[avamingli]

Allow answer query using materialized views which have external or foreign tables. Since we don't know if the data is up to date of externel table outside CBDB, introduce a new GUC:

aqumv_allow_foreign_table

Let user decide if they want to use matview instead of query on external tables.

create readable external table aqumv_ext_r(id int) 
location ('demoprot://aqumvtextfile.txt') format 'text';

create materialized view aqumv_ext_mv as
  select * from aqumv_ext_r;

explain (costs off, verbose)
select * from aqumv_ext_r;
               QUERY PLAN
------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)
   Output: id
   ->  Seq Scan on aqumv.aqumv_ext_mv
         Output: id
 Optimizer: Postgres query optimizer

Index could also be used if there were on matviews.

create index on aqumv_ext_mv(id);
explain (costs off, verbose)
select * from aqumv_ext_r where id = 5;
                            QUERY PLAN
----------------------------------------------------------------------
 Gather Motion 1:1  (slice1; segments: 1)
   Output: id
   ->  Index Only Scan using aqumv_ext_mv_id_idx on aqumv.aqumv_ext_mv
         Output: id
         Index Cond: (aqumv_ext_mv.id = 5)
 Optimizer: Postgres query optimizer

fix #ISSUE_Number

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