feat: Add Shelfmark Search when no search results

[vathzen]

Adds the functionality to search in Shelfmark ( Calibre Web Automated Book Downloader (CWABD) ) when the CWA search returns no results.
Accomplishes this by adding a new config entry config_use_shelfmark and config_shelfmark_url which can be configured in the admin panel.
The Shelfmark url is constructed from guidelines here

This is my first PR in this repo, pretty sure I might have missed a couple steps. Please let me know so that I can fix it.

I also ran scripts\update_spdx_headers.py so the number of files changed is huge. Please refer the first commit to view actual code changes for this feat.

[vathzen]

@crocodilestick do you think if this is a valuable feature? I can rebase from main to support 4.x otherwise I can close the PR 😄