Add Sensei Home plugin update notices to REST API

[jom]

Fixes #5738

Changes proposed in this Pull Request

• Adds a provider for notices to inject notices into Sensei Home's REST API endpoint.
• Adds a handler for injecting special Sensei Home notices either into Sensei_Admin_Notices (when available) or sneakily through the shared filter in the provider when Sensei_Admin_Notices isn't available (Sensei Blocks).
• Begrudgingly makes Sensei_Admin_Notices a singleton. It has a mix of classes that depend on it and would require several layers of DI.
• We planned on having this be called notifications but switched to notices to match the rest of Sensei.
• Adds the following notices:
  • When an update is available for a free plugin or licensed and activated paid plugin.
  • When an update is available for a paid plugin that isn't active an we're not sure the license status.
  • When an update is available for a paid plugin but it doesn't have an active license.

Note: This doesn't add any additional license notices yet (when a license is about to expire, for example). It was getting too big so I thought I'd wait.

Testing instructions

• Try the following with various situations: Certificates downgraded, Sensei Pro deactivated, etc.
  • You can also try this snippet to see what it is like with licensed Sensei Pro (future PR):
  • add_filter( 'sensei_home_is_plugin_licensed_sensei-pro', '__return_true' );
• Run: curl -s -X GET --user jake:jakejake 'http://sensei.local/wp-json/sensei-internal/v1/home' | jq .notices
• Confirm output makes sense.

New/Updated Hooks

• sensei_show_admin_notices_{$screen_id}: Allows hiding admin notices on a particular screen. Used to hide on Sensei Home.
• sensei_home_is_plugin_licensed_{$plugin_slug}: Allows Sensei Pro and Sensei Blocks to tell Sensei Home when their plugins have an active license.

[codecov[bot]]

Codecov Report

Merging #5900 (0e91080) into feature/new-onboarding-experience (58246cd) will increase coverage by 0.21%. The diff coverage is 76.11%.

@@                           Coverage Diff                           @@
##             feature/new-onboarding-experience    #5900      +/-   ##
=======================================================================
+ Coverage                                43.74%   43.96%   +0.21%     
- Complexity                                9134     9184      +50     
=======================================================================
  Files                                      436      441       +5     
  Lines                                    32478    32679     +201     
  Branches                                   250      252       +2     
=======================================================================
+ Hits                                     14208    14366     +158     
- Misses                                   18086    18127      +41     
- Partials                                   184      186       +2     

Impacted Files	Coverage Δ
includes/admin/class-sensei-home.php	0.00% <0.00%> (-11.87%)	:arrow_down:
...s/admin/home/class-sensei-home-guides-provider.php	100.00% <ø> (ø)
...des/admin/home/class-sensei-home-news-provider.php	100.00% <ø> (ø)
...s/admin/home/class-sensei-home-remote-data-api.php	95.08% <ø> (ø)
includes/class-sensei-admin.php	22.26% <ø> (-0.09%)	:arrow_down:
includes/class-sensei-autoloader.php	14.58% <0.00%> (-0.32%)	:arrow_down:
includes/class-sensei-data-cleaner.php	89.53% <ø> (ø)
includes/class-sensei.php	27.49% <0.00%> (-0.07%)	:arrow_down:
includes/admin/class-sensei-admin-notices.php	48.41% <35.00%> (-3.48%)	:arrow_down:
...cludes/rest-api/class-sensei-rest-api-internal.php	67.56% <50.00%> (-1.01%)	:arrow_down:
... and 17 more

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