Improve APIs for Tries in Runtime (#5756)

This is a refactor and improvement from:
https://github.com/paritytech/polkadot-sdk/pull/3881

- `sp_runtime::proving_trie` now exposes a `BasicProvingTrie` for both
`base2` and `base16`.
- APIs for `base16` are more focused on single value proofs, also
aligning their APIs with the `base2` trie
- A `ProvingTrie` trait is included which wraps both the `base2` and
`base16` trie, and exposes all APIs needed for an end to end scenario.
- A `ProofToHashes` trait is exposed which can allow us to write proper
benchmarks for the merkle trie.

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Co-authored-by: Ankan <10196091+Ank4n@users.noreply.github.com>
Co-authored-by: Adrian Catangiu <adrian@parity.io>