Implement GetMempoolInfo, GetNetworkInfo, and GetRawMempool RPC commands

[alchemistkay]

Summary

Enhance mempool and network RPC endpoints with comprehensive structured data and statistics.

Changes

• GetMempoolInfo: Returns detailed mempool statistics including transaction counts, total fees, fee rate analysis (min/max/avg), and per-shard breakdown
• GetNetworkInfo: Provides network connectivity information including peer counts, protocol version, network status, and service flags
• GetRawMempool: Returns detailed mempool entries with transaction metadata, fee rates, and shard assignments sorted by priority
• GetRawMempoolShard: Provides shard-specific mempool queries for targeted transaction analysis

Technical Implementation

• Added structured response types: MempoolInfo, NetworkInfo, RawMempoolEntry, ShardMempoolInfo
• Implemented fee rate statistics and transaction priority analysis
• Added per-shard mempool distribution and filtering capabilities
• Updated return types from String to proper Result<T, RpcErr> for structured error handling
• Integrated with existing mempool, peer info, and settings systems
• Follows existing code patterns and conventions

Test Plan

• Verify mempool statistics accuracy with various transaction loads
• Test network info reflects actual peer connections and status
• Confirm raw mempool entries are properly sorted by fee rate priority
• Test shard-specific filtering in GetRawMempoolShard
• Validate error handling for edge cases

Before/After Comparison

Before: All endpoints returned "Hello world!" stubs
After: Comprehensive mempool and network monitoring capabilities

Continues the RPC development priority outlined in the project contribution guidelines, building on PR #63.