Implement external state restoration

[puddly]

A few platforms in ZHA uses instance attributes and complex logic to derive their state, relying on Home Assistant to persist it and restore it after startup:

• I've exposed necessary information for some platforms (light and cover) for Home Assistant to store this information as extra state attributes, allowing it to be restore properly. I believe this was actually a bug in the past.
• Other platforms could be converted to use the ZCL attribute cache, such as binary_sensor and lock. These effectively perform operations that change a ZCL attribute so the attribute cache can be used to maintain state.
• Others rely on directly computing state from other information and unfortunately will have to be directly restored (siren).

In the future, we need to explore a way to store this extra state. I think zigpy can provide a hook. Alternatively, we can invert the storage logic and have ZHA implement a database driver for zigpy.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.61905% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.25%. Comparing base (6794a64) to head (b8d5c57). Report is 1 commits behind head on dev.

Files	Patch %	Lines
zha/application/platforms/number/__init__.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #58 +/- ## ========================================== + Coverage 95.00% 95.25% +0.25% ========================================== Files 61 61 Lines 9306 9335 +29 ========================================== + Hits 8841 8892 +51 + Misses 465 443 -22 ```

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[puddly]

I've foregone using the ZCL cache and instead just implement restore_external_state_attributes for the required entities. This makes this PR much simpler.