[light-client] trust ratcheting perf improvements

[alex-mysten]

Summary

Refactors trust ratcheting in the authenticated events light client:

• Parallel epoch fetching. Replaces the per-epoch sequential loop with parallel_fetch_epoch_data (buffer_unordered, concurrency 20), driven by a target epoch extracted from the proof response's certified summary.
• Single-mutex serialization. Holds the epoch_cache mutex across the entire ratchet (including network fetches). Concurrent ratchet calls now serialize: the first does the work and the rest see the cache already advanced and return immediately. Removes the previous skip-and-fresh-read dance.
• Narrower checkpoint reads. Only fetches summary.bcs and signature from the ledger service (instead of full Checkpoint with contents), and builds CertifiedCheckpointSummary directly via new_from_data_and_sig rather than going through full_checkpoint_content::Checkpoint (which requires contents to construct).
• Arc<Committee> in EpochCache to avoid expensive committee clones on every lookup.
• TLS config auto-applied to https RPC URLs.
• Hard kill-switch at MAX_RATCHET_GAP = 10_000 epochs (≈27 years) to bound work induced by malformed server responses.

Test plan

• cargo nextest run -p sui-light-client (29 passed)
• cargo simtest -p sui-e2e-tests --test authenticated_events_client_test (12 passed, including test_client_cross_epoch_stream)
• cargo xclippy -D warnings
• cargo fmt --all
• 1-off test against devnet

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