PG17 Compatibility - Fix crash when pg_class is used in MERGE

[m3hm3t]

This pull request addresses Issue #7846, where specific MERGE queries on non-distributed and distributed tables can result in crashes in certain scenarios. The issue stems from the usage of pg_class catalog table, and the FilterShardsFromPgclass function in Citus. This function goes through the query's jointree to hide the shards. However, in PG17, MERGE's join quals are in a separate structure called mergeJoinCondition. Therefore FilterShardsFromPgclass was not filtering correctly in a MERGE command that involves pg_class. To fix the issue, we handle mergeJoinCondition separately in PG17.

Relevant PG commit:
postgres/postgres@0294df2

Non-Distributed Tables:
A MERGE query involving a non-distributed table using pg_catalog.pg_class as the source may execute successfully but needs testing to ensure stability.

Distributed Tables:
Performing a MERGE on a distributed table using pg_catalog.pg_class as the source raises an error:
ERROR: MERGE INTO a distributed table from Postgres table is not yet supported
However, in some cases, this can lead to a server crash if the unsupported operation is not properly handled.

This is the test output from the same test conducted prior to the code changes being implemented.

-- Issue #7846: Test crash scenarios with MERGE on non-distributed and distributed tables
-- Step 1: Connect to a worker node to verify shard visibility
\c postgresql://postgres@localhost::worker_1_port/regression?application_name=psql
SET search_path TO pg17;
-- Step 2: Create and test a non-distributed table
CREATE TABLE non_dist_table_12345 (id INTEGER);
-- Test MERGE on the non-distributed table
MERGE INTO non_dist_table_12345 AS target_0
USING pg_catalog.pg_class AS ref_0
ON target_0.id = ref_0.relpages
WHEN NOT MATCHED THEN DO NOTHING;
SSL SYSCALL error: EOF detected
connection to server was lost

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.48%. Comparing base (c55bc8c) to head (e04858f).
Report is 2 commits behind head on release-13.0.

Additional details and impacted files

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+ Hits              53747    53752    +5     
+ Misses             4166     4165    -1     
+ Partials           2150     2149    -1