archive: Implement height, hashByHeight and call (#1582)

This PR implements:
- `archive_unstable_finalized_height`: Get the height of the most recent
finalized block
- `archive_unstable_hash_by_height`: Get the hashes (possible empty) of
blocks from the given height
- `archive_unstable_call`: Call into the runtime of a block

Builds on top of: https://github.com/paritytech/polkadot-sdk/pull/1560

### Testing Done
- unit tests for the methods with custom block tree for different
heights / forks

Closes: https://github.com/paritytech/polkadot-sdk/issues/1510
Closes: https://github.com/paritytech/polkadot-sdk/issues/1513
Closes: https://github.com/paritytech/polkadot-sdk/issues/1511

@paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>