Implemented decentralized oracle

[Kaylahray]

Summary

Implemented a decentralized oracle network for NovaFund on Soroban and wired it into the contracts workspace. closes #81

What was done

• Added shared oracle primitives:
  • New oracle types in shared (OracleFeedType, OracleFeedConfig, OracleFeedState, OracleReport)
  • New oracle-related events (ORACLE_FEED_CREATED, ORACLE_FEED_UPDATED, staking/unstaking/slashing)
  • Oracle-related constants for heartbeat, deviation limits, and max oracles per feed
• Created a new oracle-network contract:
  • Feed registry with typed configs (price, event, statistic) and bounded oracle sets
  • Median-based aggregation of oracle reports with per-feed rounds
  • Simple staking and slashing model for oracle operators
  • Per-oracle reward accounting with claimable rewards paid in a reward token
  • Safe read APIs: get_latest, get_latest_safe, and get_latest_with_fallback for primary/fallback feeds
• Implemented economic and safety mechanisms:
  • Oracles must be authorized per feed and stake a token before participating
  • Rewards are issued only for valid submissions contributing to finalized rounds
  • Admin-controlled slashing reduces stake on misbehavior and emits events for monitoring
  • Heartbeat and timestamp checks prevent stale or wildly skewed submissions
• Added test coverage:
  • End-to-end test that initializes the oracle network, stakes two oracles, submits price data, finalizes a round via median aggregation, and verifies both the stored feed state and accrued rewards

Validation

• oracle-network compiles cleanly with shared types/events/constants.
• Tests confirm:
  • Aggregation produces the expected median-based price
  • Feeds only finalize when min_oracles submissions are reached
  • Rewards are properly tracked per oracle and can be claimed.