Implement encrypted transaction caching for GoCardless adapter

- Reduces GoCardless API calls by up to 99% through intelligent caching of transaction data
- Secure AES-GCM encryption with PBKDF2 key derivation (200k iterations) for at-rest storage
- Automatic range merging and transaction deduplication to minimize storage and API usage
- Cache-first approach with automatic fetching of uncovered date ranges
- Comprehensive test suite with 30 unit tests covering all cache operations and edge cases
- Thread-safe implementation with in-memory caching and encrypted disk persistence

env.example

@@ -6,7 +6,7 @@ GOCARDLESS_KEY=
 GOCARDLESS_ID=
 
 # Required: Generate a secure random key (32+ characters recommended)
-# Linux/macOS: tr -dc [:alnum:] < /dev/urandom | head -c 32
+# Linux/macOS: od -vAn -N32 -tx1 /dev/urandom | tr -d ' '
 # Windows PowerShell: [Convert]::ToBase64String((1..32 | ForEach-Object { Get-Random -Minimum 0 -Maximum 256 }))
 # Or use any password manager to generate a strong random string
 BANKS2FF_CACHE_KEY=