feat(asap-tools): rqe query protocol and frontend switch for e2e runs #884

Part of the RQE frontend tracker. Decisions live in the tracker. Stacked on A4.

What

1. Querier (asap-tools/queriers/prometheus-client): an rqe protocol QueryClient. The query config still lists the PromQL/SQL text; the client loads a side file mapping that text to its expr tree and POSTs the tree to /api/v1/rqe_query. So result keys and calculate_fidelity stay unchanged.
2. experiment_run_e2e: Hydra switch query_engine.frontend: promql|rqe (default promql); with rqe the sketchdb server uses the rqe protocol.
3. Config writers emit the side file: sketch_bench_e2e/sketch_bench_e2e_configs.py (from the table's e2e.queries[].expr) and the random-workload artifacts.
4. End-to-end check: one classic sketch-bench table and one random workload run through the RQE route with the paced feeder; results match the PromQL route for the same plans wherever the PromQL route works.

Done when

• client + switch + side files with tests; the end-to-end check above is reported on the issue