Make triecache generic and work for no-std environments

[skunert]

This pull request refactors the trie-cache related traits and makes the system generic. Until now the caching system was bound to the LocalTrieCache implementation. This is one step towards enabling the TrieCache in validate_block https://github.com/paritytech/cumulus/issues/2432.

This refactoring contains no behavioural changes for existing code.

Change summary:

• New trait MergeableTrieCacheProvider: This trait can be implemented to provide cache implementations to the TrieBackend
• Removal of AsLocalTrieCache trait: No longer required since functionality is covered by MergeableTrieCacheProvider
• Include unimplemented cache for no std to keep compatibility. The actual cache implementation will go into cumulus.

Polkadot companion: https://github.com/paritytech/polkadot/pull/7435

[skunert]

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[skunert]

bot merge