Support Erigon Caplin consensus client

[stakepeter]

Summary

Erigon's Caplin consensus client emits a few JSON fields in non-spec-compliant forms. This PR softens the affected unmarshalers to accept both spec and Caplin forms while keeping marshal output spec-strict. The fix is purely additive — public API, marshal output, and SSZ encoding are all unchanged.

Three classes of divergence are handled:

1. Bare-number uint64 fields in execution_requests and pending_* state queues. Caplin's struct definitions in cl/cltypes/solid/{deposits,withdrawal,consolidation}.go omit the spec's ,string tag on amount, index, slot, validator_index, and withdrawable_epoch. Soften phase0.{Gwei,Slot,Epoch,ValidatorIndex}.UnmarshalJSON and electra.DepositRequest's JSON parsing to accept both "42" (spec) and 42 (Caplin). Marshal still emits "42".
2. previous_epoch_participation / current_epoch_participation as a single hex string. Caplin's ParticipationBitList.MarshalJSON emits the raw SSZ encoding as "0x..."; the spec requires a JSON array of decimal-string flags. Added altair.ParseParticipationFlags / MarshalParticipationFlags helpers and wired them into the altair / bellatrix / capella / deneb / electra / fulu BeaconState JSON paths. Accepts both forms; emits spec form.
3. Provider detection. Added a caplin case to multi.Service's provider detection so the multi-client recognizes Caplin nodes.

This makes downstream consumers work directly against Caplin without per-application JSON-rewriting shims.

Test plan

• go test ./... — all packages green
• go vet ./... clean; gofmt -l clean on touched files
• New positive tests for each lenience point: bare numbers for phase0.{Gwei,Slot,Epoch,ValidatorIndex}, mixed/bare/missing/invalid cases for electra.DepositRequest, hex/array/null/invalid cases for altair.ParseParticipationFlags
• Round-trip tests (TestBeaconBlockJSON/Good, TestSignedBeaconBlockYAML/Good, TestMarshalParticipationFlags_Roundtrip, TestDepositRequestUnmarshalJSON marshal-after-parse) confirm wire output is byte-identical for spec-form inputs
• Manually verified each Caplin divergence by reading the corresponding struct in github.com/erigontech/erigon's cl/ package