Feat/postgres 16 (#4761)

This adds PostgreSQL 16 as a vendored postgresql version, and adapts the
code to support this version. 
The important changes to PostgreSQL 16 compared to the PostgreSQL 15
changeset include the addition of a neon_rmgr instead of altering Postgres's
original WAL format.

Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

test_runner/regress/data/extension_test/5670669815/v16/ext_index.json

@@ -0,0 +1,7 @@
+{
+    "public_extensions": [],
+    "library_index": {
+        "TODO": "We still need PG16 extensions"
+    },
+    "extension_data": {}
+}