[FLINK-40007][postgres] Fix snapshot fetch size conf does not take effect

[JNSimba]

What is the purpose of the change

Relate pr: #2766

The Postgres incremental source uses the Debezium connector config's snapshot
fetch size (connectorConfig.getSnapshotFetchSize(), default 10240) for the
snapshot split read, instead of the Flink CDC option scan.snapshot.fetch.size
(default 1024). As a result:

1. scan.snapshot.fetch.size has no effect for the Postgres source.
2. When a snapshot chunk's row count is <= the fetch size, the JDBC
   server-side cursor returns the whole chunk in a single batch, loading every
   row of the chunk into memory at once. On wide tables (many columns / large
   rows) this can exhaust the heap and OOM.

The MySQL source already reads sourceConfig.getFetchSize() for its snapshot
read. This PR makes the Postgres snapshot read task do the same, so that
scan.snapshot.fetch.size is honored consistently across connectors.

Brief change log

• PostgresSnapshotSplitReadTask now takes the PostgresSourceConfig and uses
  sourceConfig.getFetchSize() for the snapshot select statement, instead of
  connectorConfig.getSnapshotFetchSize().

Verifying this change

This change is covered by existing Postgres source tests; the snapshot fetch
size now follows the scan.snapshot.fetch.size option.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[Mrart]

@JNSimba Please add ut cover this bug .