Add downloaded wal-g file check and some other adjustments

[Jamal-B]

Description

• Add wal-g download checksum check to avoid installing a corrupted or altered version. Moreover, the unarchiving task is adapted to the fact that the files included in the tar.gz of recent versions of wal-g have changed compared to older versions.
• Remove wal-g always ansible tags because we don't want ansible to always execute the wal-g role if we are targeting another role of the playbook.
• Set files's mode as advised by the ansible documentation.

Tests

• Change the wal-g version to upgrade or downgrade it while replacing the corresponding sha256 checksum, then validate that the desired version is correctly downloaded and installed.
• Repeat the same test but without updating the checksum and validate that an ansible error is thrown.
• Run ansible on a different tag than the one used to include the wal-g role and validate that the wal-g role is no longer executed.