Review Mozilla 1942341: [meta] Support adding signatures in PDFs
# 🔍 Bugzilla Audit <!-- Title: Review Mozilla <bugzilla-num>: <bugzilla-description> --> **Bugzilla**: https://bugzilla.mozilla.org/show_bug.cgi?id=1942341 ## Description <!-- Briefly describe why this issue needs further review --> Adds an option to insert a signature into a PDF. Needs review because one of the options is to save the signature, which may be in conflict with our disk persistence policies. I'm not sure myself. /cc @donuts <!-- Do not edit beneath this line <3 --> --- <!-- Make sure the "esr-" label is the correct version: -->
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