Tezlink : Support block_hash RPC and offset on block parameter

What

Extend the Tezlink backend to support block hashes retrieval /chains/main/blocks/head~2/hash

Heavily inspired by this MR : !17668 (merged)

Why

This MR introduces the ability to retrieve block hashes

How

• Imported Block_services.S.hash and exposed it through Tezlink.
• Implemented the hash method.
• Registered the new hash RPC in tezos_services.ml with appropriate conversions.