feat: customize signature stamp

[vitormattos]

Administration settings page of LibreSign about this feature:

close https://github.com/LibreSign/libres

✅ How to perform smoke tests using GitHub Codespaces

1. Open the Codespace

• Authenticate to GitHub
• Go to the branch: feat/customize-signature-stamp
• Click the Code button and select the Codespaces tab.
• Click "Create codespace on feat/customize-signature-stamp"

2. Wait for the environment to start

• A progress bar will appear on the left.
• After that, the terminal will show the build process.
• Wait until you see the message:

  ✍️ LibreSign is up!

  This may take a few minutes.

3. Access LibreSign in the browser

• Open the Ports tab (next to the Terminal).
• Look for the service running on port 80.
• Hover over the URL and click the globe icon 🌐 to open it in your browser.

4. (Optional) Make the service public

• If you want to share the app with people not logged in to GitHub, you must change the port visibility:
  • Click the three dots ⋮ on the row for port 80.
  • Select Change visibility → Public.

5. Login credentials

• Username: admin
• Password: admin

Done! 🎉
You're now ready to run smoke tests on LibreSign.

Settings

• After login, go to Administration Settings > LibreSign
• Scroll down to Collect signers' metadata
• check the option to collect and refresh the screen
• Scroll down to Signature stamp
• Configure as you wish.

to-do

• feat: custom signature template #4769
• feat: custom signature background image #4771
• don't request visible signature when haven't a signature template neither background image
• email as template var
• Allow to use "signer name" instead of signature image
• Don't request to create signature when the display mode is only the signer name
• Customize default width and height
• validate signature template with non UTF-8 chars
  - [ ] Custom font file to remove system font dependency
  Isn't possible customize the font family used by JSignPdf for now.