Add admin menu browser persistence (via schema)

[getdave]

This PR addresses #45876 by adding the required schema and HO reducer to enable browser persistence for menus portion of the adminMenu state slice.

This ensures that once the initial API response is cached in the browser IndexDB there is no (less of a) perceivable lag between Calypso UI "load" and the admin menu being "ready" and "available".

Note: this will require that #45939 is merged before we can merge this PR. Now merged 🥳

Changes proposed in this Pull Request

• Adds a schema.json for the menus portion of the adminMenu state.
• Wraps the menus portion of the adminMenu state with the schema validation reducer to enable browser peristence of state.

Testing instructions

• Checkout this PR.
• Apply blog sticker nav-unification to test site.
• Run calypso - yarn && ENTRY_LIMIT=entry-main SECTION_LIMIT=home yarn start
• Visit http://calypso.localhost:3000/?flags=nav-unification.
• You should see the menu rendered from the static data as per Nav Unification - add scaffolding for admin menu experience from static data #45836
• Once the API request completes you should see the nav re-rendered using the "correct" data from the API endpoint.
• Now reload your browser - this time you should not see the nav rendered from the static data but rather rendered semi-instantly from the cached data in the browser storage.
• Open DevTools. Goto Application tab. Look for Storage in the left hand sidebar. Expand the IndexDB accordion. Find the calypso_store DB entry. Look for the adminMenu slice of state persisted in the DB. Make sure it matches what is stored in state (try state.adminMenu in the Console tab).

Fixes #45876

[matticbot]

Test live: https://calypso.live/?branch=add/admin-menu-browser-persistence

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Async-loaded Components (~137 bytes added 📈 [gzipped])

name                                          parsed_size           gzip_size
async-load-my-sites-sidebar-unified-switcher       +413 B  (+0.2%)     +137 B  (+0.2%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[mreishus]

This works properly in my testing. I noticed something strange, but I think it's a limitation of firefox more than anything. In the firefox devtools, if you check the value of the redux-state-1234:adminMenu key, you'll only see the first 10,000 characters of the information. The rest is truncated. Additionally, the "parsed value" shows only a string, because the json is truncated and can't be parsed. However, the feature continues to work correctly, and the indexedDb info looks fine in chrome, so I suspect it's only a visual issue with Firefox's dev tools.

[cpapazoglou]

works as expected! Huge UX improvement! Thanks

[cpapazoglou]

I re-ran some failing tests