Add downgrade scripts for neon extension. (#7065)

## Problem

When we start compute with newer version of extension (i.e. 1.2) and
then rollback the release, downgrading the compute version, next compute
start will try to update extension to the latest version available in
neon.control (i.e. 1.1).

Thus we need to provide downgrade scripts like neon--1.2--1.1.sql

These scripts must revert the changes made by the upgrade scripts in the
reverse order. This is necessary to ensure that the next upgrade will
work correctly.

In general, we need to write upgrade and downgrade scripts to be more
robust and add IF EXISTS / CREATE OR REPLACE clauses to all statements
(where applicable).

## Summary of changes
Adds downgrade scripts.
Adds test cases for extension downgrade/upgrade. 

fixes #7066

This is a follow-up for
https://app.incident.io/neondb/incidents/167?tab=follow-ups

Signed-off-by: Alex Chi Z <chi@neon.tech>
Co-authored-by: Alex Chi Z <iskyzh@gmail.com>
Co-authored-by: Anastasia Lubennikova <anastasia@neon.tech>

pgxn/neon/Makefile

@@ -21,7 +21,7 @@ SHLIB_LINK_INTERNAL = $(libpq)
 SHLIB_LINK = -lcurl
 
 EXTENSION = neon
-DATA = neon--1.0.sql neon--1.0--1.1.sql neon--1.1--1.2.sql neon--1.2--1.3.sql
+DATA = neon--1.0.sql neon--1.0--1.1.sql neon--1.1--1.2.sql neon--1.2--1.3.sql neon--1.3--1.2.sql neon--1.2--1.1.sql neon--1.1--1.0.sql
 PGFILEDESC = "neon - cloud storage for PostgreSQL"
 
 EXTRA_CLEAN = \

pgxn/neon/neon--1.1--1.0.sql

@@ -0,0 +1,6 @@
+-- the order of operations is important here
+-- because the view depends on the function
+
+DROP VIEW IF EXISTS neon_lfc_stats CASCADE;
+
+DROP FUNCTION IF EXISTS neon_get_lfc_stats CASCADE;

pgxn/neon/neon--1.2--1.1.sql

@@ -0,0 +1 @@
+DROP VIEW IF EXISTS NEON_STAT_FILE_CACHE CASCADE;

pgxn/neon/neon--1.3--1.2.sql

@@ -0,0 +1 @@
+DROP FUNCTION IF EXISTS approximate_working_set_size(bool) CASCADE;

test_runner/regress/test_neon_extension.py

@@ -29,3 +29,34 @@ def test_neon_extension(neon_env_builder: NeonEnvBuilder):
             log.info(res)
             assert len(res) == 1
             assert len(res[0]) == 5
+
+
+# Verify that the neon extension can be upgraded/downgraded.
+def test_neon_extension_compatibility(neon_env_builder: NeonEnvBuilder):
+    env = neon_env_builder.init_start()
+    env.neon_cli.create_branch("test_neon_extension_compatibility")
+
+    endpoint_main = env.endpoints.create("test_neon_extension_compatibility")
+    # don't skip pg_catalog updates - it runs CREATE EXTENSION neon
+    endpoint_main.respec(skip_pg_catalog_updates=False)
+    endpoint_main.start()
+
+    with closing(endpoint_main.connect()) as conn:
+        with conn.cursor() as cur:
+            all_versions = ["1.3", "1.2", "1.1", "1.0"]
+            current_version = "1.3"
+            for idx, begin_version in enumerate(all_versions):
+                for target_version in all_versions[idx + 1 :]:
+                    if current_version != begin_version:
+                        cur.execute(
+                            f"ALTER EXTENSION neon UPDATE TO '{begin_version}'; -- {current_version}->{begin_version}"
+                        )
+                        current_version = begin_version
+                    # downgrade
+                    cur.execute(
+                        f"ALTER EXTENSION neon UPDATE TO '{target_version}'; -- {begin_version}->{target_version}"
+                    )
+                    # upgrade
+                    cur.execute(
+                        f"ALTER EXTENSION neon UPDATE TO '{begin_version}'; -- {target_version}->{begin_version}"
+                    )