Move optimizer_enable_query_parameter GUC to Orca extension

[whitehawk]

Move optimizer_enable_query_parameter GUC to Orca extension

Problem:
'optimizer_enable_query_parameter' GUC is a setting that affects Orca behavior,
so we would like it to reside in Orca. But this GUC was also referenced by the
core (in 'choose_custom_plan()'). It prevented moving of the GUC to the
extension.

Fix:
In 'choose_custom_plan()' the GUC was used to make a decision whether to use
custom or generic plan in plan cache manager. In general, this decision can be
affected by different parameters, and a generic external planner should be able
to have control over this decision. Thus, this patch:

• Adds a hook, that is checked in 'choose_custom_plan()'.
• Moves the custom logic, involving 'optimizer_enable_query_parameter' GUC,
  to the extension side.
• Moves 'optimizer_enable_query_parameter' GUC into the extension.
• Also, it removes some names from 'unsync_guc_name.h', that were moved to the
  extension previously.

[andr-sokolov]

Can we revert 38f4cb8 instead of replacing GUC with hook? The purpose is to make it simpler bumping PostgreSQL version, to reduce difference between PostgreSQL and GPDB, but the diff in choose_custom_plan() is increased.