[Global Styles] Implement Global Styles endpoint

[chipsnyder]

Description

Fixes wordpress-mobile/gutenberg-mobile#3163
Related PRs

• WordPress-iOS [Global Styles] Implement Global Styles endpoint WordPress-iOS#16411
• Gutenberg-Mobile [Global Styles] Adds new Prop for Global Styles Settings gutenberg-mobile#3459
• Gutenberg [RN Mobile][Global Styles] Adds new Prop for Global Styles Settings WordPress/gutenberg#30544

Tests:

📓 This can be tested along side wordpress-mobile/WordPress-iOS#16411

Regression Tests:

These should be run on a site with "Global Style Settings" disabled or a site that doesn't have WordPress 5.8 installed:

• Default Colors - Check that default colors still load - steps
• Default Gradients - Check that default gradients still load - steps
• Custom Colors - Check that custom colors load in the editor - steps
• Custom Gradients - Check that custom gradients load in the editor - steps
• Offline Support - steps

Tests:

These should be run on a site with "Global Style Settings" enabled and a site that has WordPress 5.8 installed:

• FSE Colors and Gradients - steps
• Offline Support - steps

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• Please check here if your pull request includes additional test coverage.

[antonis]

Great work Chip 👍
Tested this via the iOS PR and works as expected. The code also looks great 🎉