Dac: observer client stub

Context

To smoothen integration with rollup node and ease with testing, an observer client stub is prepared. The t type is garbage at the moment. Please see from !8163 (closed) to get an idea of what it will be rewritten into. The important thing here is that it is an abstract type from a caller's point of view.

The rollup team would implement a cli to parse and initialize the client. Then the remaining implementation would follow from !8163 (closed). This 2 step process will allow us to test the implementation.

Part of #5199 (closed)

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