Add mergepdf action

[joecorall]

This PR adds the Drupal Action needed to create PDFs for paged content Islandora-Devops/isle-buildkit#501

I sort of don't want to merge this without getting Islandora-Devops/isle-site-template#98 merged first and then creating a new mergepdf service after that PR merges. Otherwise we'd need to create a dev and prod version of this new mergepdf service (which is a little absurd)

The proper order is probably:

• merge Default dev to http. Replace dev/prod profiles with compose override file isle-site-template#98
• create the mergepdf service in https://github.com/Islandora-Devops/isle-site-template

services:
  mergepdf:
    image: islandora/mergepdf:6.2.0
    secrets: *secrets-jwt-public

• merge this PR