[YSQL] Consolidate DML access to pggate functions

Jira Link: [DB-6686](https://yugabyte.atlassian.net/browse/DB-6686)
### Description

There is a lot of duplication around DML code interfacing with pggate. The duplication is generally across access methods and scan types:

- ForeignScan
- lsm YbSeqScan, IndexScan, IndexOnlyScan
- ybgin IndexScan, IndexOnlyScan

Such duplication has led to fixes being implemented on one copy and not the others. For example, commit 0a154aaa28787626c759fa8110959506e58c9282 followed by commit 23143a24b99151ef1a90b21dbfb989c9c05c3785. Or some features being present in one copy but not others, such as aggregate pushdown only in ForeignScan (#15857). Or some bugs that have yet to be discovered. For example, #17495.

Examples of duplication are ybSetupScanQual, ybSetupScanColumnRefs, ybcAddTargetColumn. This issue will track them all, and it can close when deduplication feels good enough.

See also #17462.

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[DB-6686]: https://yugabyte.atlassian.net/browse/DB-6686?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ