[FEAT] Cover block linear-gradient

[dratwas]

This is a part of #1737

In this PR i added support for the linear gradient that is set on the web in the cover block.

Gutenberg PR - WordPress/gutenberg#20411

To test:
Add below code to initial-html:

<!-- wp:cover {"minHeight":430,"gradient":"luminous-dusk"} -->
<div class="wp-block-cover has-background-dim has-luminous-dusk-gradient-background" style="min-height:430px"><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…","fontSize":"large"} -->
<p class="has-text-align-center has-large-font-size"></p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

<!-- wp:cover {"customGradient":"linear-gradient(135deg,rgb(254,205,165) 24%,rgb(254,45,45) 50%,rgb(107,0,62) 77%)"} -->
<div class="wp-block-cover has-background-dim has-background-gradient" style="background:linear-gradient(135deg,rgb(254,205,165) 24%,rgb(254,45,45) 50%,rgb(107,0,62) 77%)"><div class="wp-block-cover__inner-container"><!-- wp:paragraph {"align":"center","placeholder":"Write title…"} -->
<p class="has-text-align-center">Hello moto</p>
<!-- /wp:paragraph --></div></div>
<!-- /wp:cover -->

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[geriux]

Approved via WordPress/gutenberg#20411 (review)