Dac: introduce modes of operation

Context

The DAC node will run in three different modes of operation: Coordinator, Dac_member and Observer. It will also support a temporary Legacy mode, where the centralised DAC is run. This MR introduces the four different modes of operations. It also ensures that the Dac node can run in any of these modes with the necessary information.

Part of: #4613 (closed)

TODO:

• Rewrite init config commands to set mode of operation (!7519 (merged))
• Retrieve account keys according to mode of operation
  • Coordinator: public keys of dac_members; sk_uri of operator
  • Dac_member: sk_uri of dac_member
  • Observer: None
  • Legacy mode: public and secret keys of dac_members
• Integration tezts

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