Expose wal_fpi_bytes in EXPLAIN (WAL)

 Expose wal_fpi_bytes in EXPLAIN (WAL)
 
 The new wal_fpi_bytes counter calculates the total amount of full page
 images inserted in WAL records, in bytes.  This commit exposes this
 information in EXPLAIN (ANALYZE, WAL) alongside the existing counters,
 for both the text and JSON/YAML outputs, building upon f9a09aa29520.
 
 Author: Shinya Kato <shinya11.kato@gmail.com>
 Reviewed-by: Michael Paquier <michael@paquier.xyz>
 Discusssion: https://postgr.es/m/CAOzEurQtZEAfg6P0kU3Wa-f9BWQOi0RzJEMPN56wNTOmJLmfaQ@mail.gmail.com