Randomize leader election

[ss-es]

Closes #3684

This PR:

• Adds randomized (seeded by view) leader rotation as a separate type implementing the Membership trait. This logic previously existed but was gated behind a feature flag.
• Mostly removes feature flag gating around leader rotation logic. The randomized-leader-election feature is completely removed, while fixed-leader-election is now exclusively a feature in the examples crate.
• test_success and test_all_restart now also run with randomized leader rotations.

This PR does not:

Key places to review:

• Are there other tests that should be run with randomized leader election?

[dailinsubjam]

Others LGTM! Copy-pasted from Zullip for future sequencer-benchmarking reference:

https://github.com/EspressoSystems/espresso-sequencer/blob/8151b2b54c36802d13ef7262a6d364176ae13509/sequencer/src/context.rs#L106-L110

the first argument to new is just the nodes that you want to allow to be leader.

[bfish713]

Why not just name this GeneralRandomCommittee? or similar so we don't have 2 structs named the exact same thing, I could see this causing some weird headache in the future. Unless this is some common code style thing I wasn't aware of

[ss-es]

I was just following the style from before, but yeah I agree -- renamed!