SCORU,Node: multiple (optional) operators and modes

Context

Fixes #2882 (closed).

Based on !5955 (merged).

The command to configure the rollup node is now:

  init <mode> config for <sc-rollup-address> with operators [<operator>...]
    <mode>: The mode for the rollup node (observer, batcher, maintenance, operator, custom)
    <sc-rollup-address>: The smart-contract rollup address
    <operator>: Public key hash, or alias, of a smart-contract rollup node operator. An operator can be specialized to a particular purpose by prefixing it's key or alias by said purpose, e.g. publish:alias_of_my_operator.

The modes are:

• Observer: Only follows the chain, reconstructs and interprets inboxes
• Batcher: Accept transactions in its queue and batches them on the L1 (TODO)
• Maintenance: Follows the chain and publishes commitments, cement and refute
• Operator: Equivalent to maintenance + batcher
• Custom: This mode allows to tweak which operations are injected by selecting the operators/signers

Manually testing the MR

dune exec tezt/tests/main.exe -- --file sc_rollup.ml commitment -i -t 'Alpha: observing the correct handling of commitments in the rollup node (handles_chain_reorgs)'

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