Proto: add new arguments to the 'list delegates' RPC

The RPC ../context/delegates takes two additional Boolean flags with_minimal_stake and without_minimal_stake, which allow to enumerate only the delegates that have at least a minimal stake to participate in consensus and in governance, or do not have such a minimal stake, respectively.

Addresses #2171 (closed).

Manually testing the MR

To test this in sandbox mode:

• create a new pair of keys alice
• transfer a small amount (less than one roll) to alice
• register alice as a delegate
• run the RPC with/without the newly introduced arguments
• check alice's pkh in the different outputs

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR