Rename CachedPlanType to PlannedStmtOrigin for PlannedStmt

Commit 719dcf3c42 introduced a field called CachedPlanType in
PlannedStmt to allow extensions to determine whether a cached plan is
generic or custom.

After discussion, the concepts that we want to track are a bit wider
than initially anticipated, as it is closer to knowing from which
"source" or "origin" a PlannedStmt has been generated or retrieved.
Custom and generic cached plans are a subset of that.

Based on the state of HEAD, we have been able to define two more
origins:
- "standard", for the case where PlannedStmt is generated in
standard_planner(), the most common case.
- "internal", for the fake PlannedStmt generated internally by some
query patterns.

This could be tuned in the future depending on what is needed.  This
looks like a good starting point, at least.  The default value is called
"UNKNOWN", provided as fallback value.  This value is not used in the
core code, the idea is to let extensions building their own PlannedStmts
know about this new field.

Author: Michael Paquier <michael@paquier.xyz>
Co-authored-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/aILaHupXbIGgF2wJ@paquier.xyz

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