Test (wal-g) backup compression algorithms are supported

Why?

Wal-g 3.x made brotli compression an optional build flag, see https://github.com/wal-g/wal-g/pull/1379. Upgrading to v3.x broke assumptions in some clusters relying on brotli compression and explicitly setting backup creation to use wal-g. Restoration would have equally failed, but in those cases no backups could be created in the first place.

Proposed solution

• The proposed fix is to use pre-built wal-g binaries from the source repository itself instead of compiling in Spilo, which at this moment seem to include brotli compression (https://github.com/zalando/spilo/pull/1006)

To prevent any regressions, we should run automated tests to verify that all expected compression algorithms are still working for each Spilo build as expressed in this comment https://github.com/zalando/spilo/pull/1006#issuecomment-2244915865, because lz4 or lzma might be dropped (or made optional) at one time in future as well.

This would help detecting issues with creating or restoring from backups early.

for algo in lz4 lzma brotli; do
  # ... verify backups can be created and restored from $algo
done

Start with the proposed pre-built wal-g binary change in Spilo PR #1006 and its referenced comment, then locate the existing backup creation and restoration test entry points. Add automated coverage for lz4, lzma, and brotli across each Spilo build, verifying that backups can be created and restored with every algorithm.