feat: add remote materialization store for Go

[nicklasl]

Implements gRPC-based remote materialization storage to support sticky assignments and materialized segments without requiring custom infrastructure.

Key Changes:

• Proto updates: Added WriteMaterializedOperations and ReadMaterializedOperations RPCs to internal_api.proto
• RemoteMaterializationStore: New implementation that stores materialization data via gRPC to Confidence service with client secret authentication
• One-line setup: Enable with UseRemoteMaterializationStore: true in ProviderConfig
• Configurable timeouts: Separate env vars for read (default: 2s) and write (default: 5s) operations
  • CONFIDENCE_MATERIALIZATION_READ_TIMEOUT_SECONDS
  • CONFIDENCE_MATERIALIZATION_WRITE_TIMEOUT_SECONDS
• Comprehensive tests: 11 integration tests covering error propagation, timeouts, auth failures, and happy paths

Usage:

  provider, err := confidence.NewProvider(ctx, confidence.ProviderConfig{
      ClientSecret: "your-client-secret",
      UseRemoteMaterializationStore: true,
  })