[Epic] Cluster-specific `Optimize` pipelines

[aalexandrov]

Product outcome

Materialize engineers working on changes in the compute layer that require changes to the Optimize pipelines can perform production experiments on unbilled clusters which force the proposed new version of the pipeline (enabled behind a cluster-specific feature flag).

Discovery

• Design doc PR: https://github.com/MaterializeInc/materialize/pull/25010
• MVP PR: https://github.com/MaterializeInc/materialize/pull/25165

Work items

### Pre-production (discovery)
- [x] #25010
- [x] #25165

### Production (development)
- [ ] https://github.com/MaterializeInc/materialize/pull/25373
- [ ] https://github.com/MaterializeInc/materialize/pull/25544
- [ ] https://github.com/MaterializeInc/materialize/pull/25586
- [ ] https://github.com/MaterializeInc/materialize/pull/25670
- [ ] https://github.com/MaterializeInc/materialize/pull/25811
- [ ] https://github.com/MaterializeInc/materialize/pull/25812
- [ ] https://github.com/MaterializeInc/materialize/pull/25628
- [ ] https://github.com/MaterializeInc/materialize/pull/26460
- [ ] Add support for `EXPLAIN SUBSCRIBE` (stretch goal)
- [ ] Add support for `EXPLAIN COPY TO` (stretch goal)

### Post-production (rollout)
- [x] Merge design doc that reflects the merged version of the code
- [x] Use it to run `enable_eager_delta_joins` experiments
- [x] Use it to run `enable_variadic_outer_joins` experiments

Decision log

• 15 February 2024. Putting active development on hold in order to prioritize #24155.
• 1 February 2024. Initiated the discovery phase.

[ggevay]

This really seems done.