Proto: small updates to the delegates/<pkh>/ RPCs

This MR aims to improve the delegates/<pkh>/... RPCs.

It also fixes a small bug, where for the first Tenderbake level the validators are considered not participating, when they in fact are not required to participate.

The MR is best reviewed commit by commit.

How to test

In sandbox mode, let only 4 out of the 5 bootstrap accounts bake, and see how the participation info evolves.

Checklist

• [N/A] Document the interface of any function added or modified (see the coding guidelines)
• [N/A] 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