Proto/SWRR: integration of SWRR under feature flag

What

TODO: protocol migration test.

Changes in select_distribution_for_cycle, baking_rights_owner and cycle_end functions to include SWRR logic under feature flag.

Why

How

Using feature flag to choose between old and new sampling method at cycle end. For baking_rights_owner we check first that the new method is available, and default to the old method if it is not.

Manually testing the MR

In sandbox mode, use RPCs get /chains/main/blocks/head/helpers/swrr_credits and get /chains/main/blocks/head/helpers/swrr_selected_bakers.

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