It turns out that our main regression test suite queries tables upon which concurrent DDL is occurring, which can, rarely, cause test_plan_advice failures. We're not quite ready to fix that problem just yet, because we want to gather some more information about how often it actually happens first. But, our plan is going to require test_plan_advice to access a few bits of pg_plan_advice that have been considered internal up until now, so this commit rejiggers things to expose those bits.

First, test_plan_advice is going to need to be able to interpret the PGPA_TE_* constants which have been declared in pgpa_trove.h. The "TE" stands for "trove entry" but that's kind of a silly name; change the naming to "FB" (for "feedback") and move the declarations to pg_plan_advice.h, which is a header file that's already installed. This has the side benefit of making these constants available to any other extensions that may want to examine plan advice feedback.

Second, test_plan_advice is going to call pgpa_planner_feedback_warning, so make that function non-static and mark it PGDLLEXPORT.

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