This PR lays the foundation for implementing the archive RPC methods.

The methods implemented by this PR:
- archive_unstable_body: Fetch the block's body (a vector of hex-encoded
scale-encoded extrinsics) from a given block hash
- archive_unstable_genesisHash: Fetch the genesis hash
- archive_unstable_header: Fetch the header from a given block hash

Added unit tests for the methods.

This PR is implementing the methods without exposing them to the RPC
layer; which are to be exposed by a follow-up PR.

Closes: https://github.com/paritytech/polkadot-sdk/issues/1509 
Closes: https://github.com/paritytech/polkadot-sdk/issues/1514

@paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>