chainHead: Add support for storage closest merkle descendant

[lexnv]

This PR adds support for fetching the closest merkle value of some key.

Builds on top of

• https://github.com/paritytech/substrate/pull/14755
• https://github.com/paritytech/trie/pull/199

Closes: https://github.com/paritytech/substrate/issues/14550

// @paritytech/subxt-team

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs. Job name: cargo-check-benches Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3430121