Dac: Rollup integration - dac observer client

Context

Observer client for interacting with an observer node that can be used by the rollup node. The Observer client exposes the function fetch_missing_page that does conversion between Sc_rollup_reveal_hash and Dac_hash, calls the equivalent function on Dac_node_client then verifies the hash.

It is assume that the observer is running locally. The behaviour of the client is documented in the design doc: https://hackmd.io/PBNbp7gfRbevwdnCjMlmng#The-rollup-node.

Depends on: !8007 (merged) Closes: #4524 (closed)

Manually testing the MR

dune exec tezt/tests/main.exe -- --file dac.ml

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR