Use FDW to query multiple servers as shards

[oppenheimer01]

This commit mainly meets the needs of users to query multiple clusters as external shards. fdw treats each data source as a whole without knowing its internal structure. It keeps requesters and data sources properly decoupled to maintain generality.

Add a new catalog table pg_foreign_table_seg to enable multiple shards in foreign table. The foreign table should be treated as a shard table with strewn locus. Each QE scanning the foreign should got a shard from pg_foreign_table_seg.

Considering that the size of the computing cluster and the number of shards of the foreign table may be inconsistent. Use flexible gang to generate the same number of scan nodes as foreign table shards.

Considering that the data bandwidth between different data centers is limited, we need to reduce the data transmission of fdw as much as possible. Pushing the execution node down to the remote end as much as possible can reduce data transmission.

If all tables of a subtree are distributed in the same foreign server collection, It can be pushed down. But in mpp-fdw, we should consider if a table only joinning the shared in same foreign server. So a new system attribute gp_foreign_server was add to the foreign table. If the customer add "t1.gp_foreign_server = t2.gp_foreign_server" to join condition. It should be pushed down.

We can only push down the first stage of the two-stage aggregation. Multi-stage aggregation will use some intermediate types. Some of these intermediate types are external types that can be output externally, such as count, min, max, and sum. The intermediate and final types of these types are identical. Others are more complex internal types, such as avg, whose intermediate type is inconsistent with the final type and must be converted using a final function. Since the local node in FDW serves as a standard client to exchange data with the remote server, these internal types cannot be transmitted. So some of the aggregate functions such as "avg" should not be pushed down now.

fix #ISSUE_Number

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[avamingli]

Hi, @oppenheimer01 thanks for your contribution!
Awesome codes and rich description. I'll review this pr soon.

[yjhjstz]

ic-singlenode-test some errors.

[oppenheimer01]

ic-singlenode-test some errors.

Got it.