impr(ci): Refactor postgres Makefile targets to a separate makefile (#12363)

Mainly for general readability. Some notable changes:

- Postgres can be built without the rest of the repository, and in
particular without any of the Rust bits. Some CI scripts took advantage
of that, so let's make that more explicit by separating those parts.
Also add an explicit comment about that in the new postgres.mk file.

- Add a new PG_INSTALL_CACHED variable. If it's set, `make all` and
other top-Makefile targets skip checking if Postgres is up-to-date. This
is also to be used in CI scripts that build and cache Postgres as
separate steps. (It is currently only used in the macos walproposer-lib
rule, but stay tuned for more.)

- Introduce a POSTGRES_VERSIONS variable that lists all supported
PostgreSQL versions. Refactor a few Makefile rules to use that.

.github/workflows/build-macos.yml

@@ -135,6 +135,12 @@ jobs:
           name: pg_install--v17
           path: pg_install/v17
 
+      # `actions/download-artifact` doesn't preserve permissions:
+      # https://github.com/actions/download-artifact?tab=readme-ov-file#permission-loss
+      - name: Make pg_install/v*/bin/* executable
+        run: |
+          chmod +x pg_install/v*/bin/*
+
       - name: Cache walproposer-lib
         id: cache_walproposer_lib
         uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4.2.3
@@ -162,7 +168,7 @@ jobs:
       - name: Build walproposer-lib (only for v17)
         if: steps.cache_walproposer_lib.outputs.cache-hit != 'true'
         run:
-          make walproposer-lib -j$(sysctl -n hw.ncpu)
+          make walproposer-lib -j$(sysctl -n hw.ncpu) PG_INSTALL_CACHED=1
 
       - name: Upload "build/walproposer-lib" artifact
         uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2