Add so files from the specified version to the test image

[eaydingol]

Add the library files from the latest minor release to exttester image.
These files will be used in mixed version testing, see draft PR citusdata/citus#8369

Tested locally for 17.7

make build-exttester-17.7
// in container
ls /opt/citus-versions/v13.2.0/
citus.so  citus_columnar.so  citus_decoders  citus_pgoutput.so  citus_wal2json.so

Slight increase in the image size:

ghcr.io/citusdata/exttester            17.7-dev-15cd08f      ae73e573b28d   7 seconds ago   3.02GB   
ghcr.io/citusdata/exttester            17.7-dev-076d6fb      be3fc5177d09   3 minutes ago   3.05GB  // from this PR

See related PR in citus repo, testing the dev image as well : citusdata/citus#8390