Create automated script for ESR bumps

[sagudev]

Every once in a while we need to do SM bump (still in same ESR channel so only security updates) to keep integrity check happy: #566

Currently this involves some manual labor, that I would like to automate into scheduled run:

• ./mozjs-sys/etc/get_latest_mozjs.sh
• python3 ./mozjs-sys/etc/update.py mozjs.tar.xz
• git commit -m "Apply patches"
• check task id at https://treeherder.mozilla.org/#/jobs?repo=mozilla-esr128&filter-searchStr=spidermonkey%20pkg to obtain commit and update COMMIT file
• set mozjs-sys version to ESRs version

last two steps needs to be scripted before we can automate whole workflow.