systemd-storage-block: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The abstract is changed from the noun phrase
"Simple block device backed storage provider" to the imperative "Expose
local block devices as storage volumes.", following the man page and
consistent with the other commands.

test/units/TEST-74-AUX-UTILS.introspect-cli.sh

@@ -51,6 +51,7 @@ INTROSPECTABLE=(
     systemd-socket-activate
     systemd-socket-proxyd
     systemd-stdio-bridge
+    systemd-storage-block
     systemd-tty-ask-password-agent
     systemd-veritysetup
     timedatectl