Prepare pg 15 support (build system and submodules) (#2337)

* Add submodule postgres-15

* Support pg_15 in pgxn/neon

* Renamed zenith -> neon in Makefile

* fix name of codestyle check

* Refactor build system to prepare for building multiple Postgres versions.

Rename "vendor/postgres" to "vendor/postgres-v14"

Change Postgres build and install directory paths to be version-specific:

- tmp_install/build -> pg_install/build/14
- tmp_install/* -> pg_install/14/*

And Makefile targets:

- "make postgres" -> "make postgres-v14"
- "make postgres-headers" -> "make postgres-v14-headers"
- etc.

Add Makefile aliases:

- "make postgres" to build "postgres-v14" and in future, "postgres-v15"
- similarly for "make postgres-headers"

Fix POSTGRES_DISTRIB_DIR path in pytest scripts

* Make postgres version a variable in codestyle workflow

* Support vendor/postgres-v15 in codestyle check workflow

* Support postgres-v15 building in Makefile

* fix pg version in Dockerfile.compute-node

* fix kaniko path

* Build neon extensions in version-specific directories

* fix obsolete mentions of vendor/postgres

* use vendor/postgres-v14 in Dockerfile.compute-node.legacy

* Use PG_VERSION_NUM to gate dependencies in inmem_smgr.c

* Use versioned ECR repositories and image names for compute-node.
The image name format is compute-node-vXX, where XX is postgres major version number.
For now only v14 is supported.
Old format unversioned name (compute-node) is left, because cloud repo depends on it.

* update vendor/postgres submodule url (zenith->neondatabase rename)

* Fix postgres path in python tests after rebase

* fix path in regress test

* Use separate dockerfiles to build compute-node:
Dockerfile.compute-node-v15 should be identical to Dockerfile.compute-node-v14 except for the version number.
This is a hack, because Kaniko doesn't support build ARGs properly

* bump vendor/postgres-v14 and vendor/postgres-v15

* Don't use Kaniko cache for v14 and v15 compute-node images

* Build compute-node images for different versions in different jobs

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

.dockerignore

@@ -13,6 +13,7 @@
 !pgxn/
 !proxy/
 !safekeeper/
-!vendor/postgres/
+!vendor/postgres-v14/
+!vendor/postgres-v15/
 !workspace_hack/
 !neon_local/