[Global Styles] Implement Global Styles endpoint

[geriux]

Description

Once the new Global styles API endpoint (WordPress/gutenberg#29063) is added in Gutenberg, it needs to be implemented in the main WordPress iOS and Android apps.

As a reference, this work is very similar to the one done before about theme colors. Where the main apps would handle fetching the data, store it for offline support, and passing it to the editor.

Goals

• The editor should have the Global styles settings available to use.