DAC: Refactor Dac node client into its own library

Context

This MR puts Dac_node_client into its own library lib_dac_client. To do this cleanly, dependent repr types and rpc definitions are factored out into their own library lib_dac. lib_dac_client allows us to abstract away the implementation of fetching preimages from the user (for example, using a side car service). At the moment, lib_dac_client is a simple wrapper over DAC node rpcs which is enough for to integrate into the rollup.

Manually testing the MR

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