TryDecodeEntireState check for storage types and pallets (#1805)

### This PR is a port of this [PR for
substrate](https://github.com/paritytech/substrate/pull/13013) by
@kianenigma

Add infrastructure needed to have a Pallet::decode_entire_state(), which
makes sure all "typed" storage items defined in the pallet are
decode-able.

This is not enforced in any way at the moment. Teams who wish to
integrate/use this in the try-runtime feature flag should add
frame_support::storage::migration::EnsureStateDecodes as the LAST ITEM
of the runtime's custom migrations, and pass it to frame-executive. This
will make it usable in try-runtime on-runtime-upgrade.

This now catches cases like
https://github.com/paritytech/polkadot-sdk/pull/1969:
```pre
ERROR runtime::executive] failed to decode the value at key: Failed to decode value at key: 0x94eadf0156a8ad5156507773d0471e4ab8ebad86f546c7e0b135a4212aace339. Storage info StorageInfo { pallet_name: Ok("ParaScheduler"), storage_name: Ok("AvailabilityCores"), prefix: Err(Utf8Error { valid_up_to: 0, error_len: Some(1) }), max_values: Some(1), max_size: None }. Raw value: Some("0x0c010101010101")
```

... or:

![image](https://github.com/paritytech/polkadot-sdk/assets/10380170/73052d4f-4da5-4b21-a8dd-b17004e5965e)

Closes #241

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/support/src/storage/generator/mod.rs

@@ -24,10 +24,10 @@
 //!
 //! This is internal api and is subject to change.
 
-mod double_map;
+pub(crate) mod double_map;
 pub(crate) mod map;
-mod nmap;
-mod value;
+pub(crate) mod nmap;
+pub(crate) mod value;
 
 pub use double_map::StorageDoubleMap;
 pub use map::StorageMap;