Add new swh-toolbox application

This is a deployment toolbox. For starters, it will ease the registering of new scheduler task type.

• app-manager built generated its frozen requirements without issue (locally)
• Then docker built the image

$ grep swh apps/swh-toolbox/requirements-frozen.txt
swh.core==2.24.0
swh.counters==0.9.2
swh.journal==1.3.6
swh.lister==5.9.8
swh.loader.core==5.10.0
swh.model==6.8.0
swh.objstorage==2.3.0
swh.perfecthash==1.0.0
swh.scheduler==1.10.1
swh.storage==1.16.0
$ docker build .
...
Step 11/11 : ENTRYPOINT "/opt/swh/entrypoint.sh"
 ---> Running in 630d8f13e6d3
Removing intermediate container 630d8f13e6d3
 ---> 1e19f56f1cc1
Successfully built 1e19f56f1cc1

TODO: In another mr, add the swh-charts part to have an appropriate scheduler configuration file the 'swh scheduler' cli could use.

Refs. sysadm-environment#5030 (closed)