Form Block: Add Margin, Padding and Background Color Options

[n7studios]

Summary

Adds margin, padding and background color options to the Form Block:

Testing

• testFormBlockWithThemeColorParameters: Test the Form block's theme color parameters works.
• testFormBlockWithHexColorParameters: Test the Form block's hex color parameters works.
• testFormBlockWithMarginAndPaddingParameters: Test the Form block's margin and padding parameters works.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)