Add Legal Signing field support

[jakejackson1]

Description

This PR adds support for the Legal Signature (both text and image-based signatures) and Legal Consent fields in Gravity PDF.

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0.76336% with 130 lines in your changes are missing coverage. Please review.

Project coverage is 76.18%. Comparing base (cde4341) to head (3e92228).

Files	Patch %	Lines
src/Helper/Fields/Field_Fg_Ls_Signature.php	0.00%	50 Missing :warning:
src/View/html/PDF/add_legalsigning_styles.php	0.00%	46 Missing :warning:
src/Model/Model_PDF.php	0.00%	16 Missing :warning:
src/Helper/Fields/Field_Fg_Ls_Consent.php	0.00%	15 Missing :warning:
src/Controller/Controller_PDF.php	25.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1515 +/- ## =============================================== - Coverage 76.96% 76.18% -0.78% =============================================== Files 244 247 +3 Lines 12739 12870 +131 Branches 370 370 =============================================== + Hits 9804 9805 +1 - Misses 2927 3057 +130 Partials 8 8 ```

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