Add new site setting to hide the Action Bar

[ntsekouras]

Related to Automattic/wp-calypso#53830

This PR adds a new general setting (wpcom_hide_action_bar) to conditionally show the Action Bar on the front end of a site. The default value will be falsy to preserve the current behavior that Action Bar is shown.

The main issue will need 2 more PRs to be complete that I'll create and link shortly:

1. 169996-ghe-Automattic/wpcom to add the check for this option and not enqueue the Action Bar. Additionally it handles the setting in classic wp-admin interface.
2. Calypso PR that adds the UI for the default interface

Other information:

• Have you written new tests for your changes, if applicable?
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Testing instructions:

You need to test this PR with the other linked PRs.

1. Go to general settings and update the value
2. Go to the front-end and observe that the Action Bar is rendered based on that option.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the add/wpcom_hide_action_bar-site-option branch.

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[github-actions]

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[tyxla]

This looks good overall 👍

Left some suggestions.

Worth double-checking from someone from Jetpack just in case we're missing something when introducing a new field to the Jetpack/WPCOM settings API endpoints.

[tyxla]

For the protocol: I found it an odd design decision that we need to provide this option both here in the ancestor class and the v4 endpoint class.