[YSQL] Refactor yb_disable_transactional_writes code path

[huapengy]

Jira Link: DB-15499

Description

We are accessing yb_disable_transactional_writes directly in many code path. One stack:

@        ybcModifyTable.c  YBCExecuteInsertIndexForDb
@        ybcModifyTable.c  YBCExecuteInsertIndex
@        0x1045d8d5b  ybcininsert
@        0x1044ecb2f  index_insert
@        execIndexing.c  YbExecDoInsertIndexTuple
@        copyfrom.c  ExecInsertIndexTuples
@        0x104747f3f  CopyFrom

In the cases we need to change yb_disable_transactional_writes value dynamically in code, we may forget to restore back. Better to refactor the code to pass an argument like YbcPgTransactionSetting through the stack.

Issue Type

kind/enhancement

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