Add oracle median price and signer quorum config

[Tomoya0k]

Closes #280

Summary

This PR adds multi-signer oracle price aggregation with median price selection and per-token quorum configuration.

Changes

• Added per-token signer configuration and quorum storage.

• Added admin configuration for trusted keeper signer indices.

• Added median calculation for signed oracle submissions.

• Updated production set_prices(...) to:

  • validate submitted signed prices
  • preserve timestamp and ledger sequence freshness checks
  • preserve upstream circuit breaker validation
  • validate signer membership per token
  • reject duplicate signer submissions for the same token
  • verify ed25519 signatures
  • enforce quorum before storing prices
  • store median min/max prices per token

• Preserved backward compatibility when no per-token signer config is set.

• Kept set_prices_simple(...) as a simple test helper that stores submitted TokenPrice values directly while still running circuit breaker checks.

• Added/update tests for median calculation, signer configuration, quorum validation, duplicate signer rejection, and single-signer compatibility.

Tests

• cargo fmt
• cargo test -p oracle

Notes

During workspace testing, an ADL handler failure was observed. I verified that the same failure occurs when using origin/main's oracle implementation, with the error happening in apply_delta_to_u128, so it appears unrelated to this oracle change.