Dac: extend observer config with committee member rpcs.

Context

The context of this MR can be found #4522 (closed). We want missing pages to be fetched from Committee Members instead of the Coordinator.

This MR adds committee member endpoints to observer configuration. These endpoint would be used for querying missing pages.

Part 1/2 of: #4522 (closed)

Part 2/2: !8747 (merged)

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).
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