fix: use @preconcurrency import for Swift 6.2 Crypto compatibility

- Replace @unchecked Sendable with @preconcurrency import Crypto
- Cleaner approach that doesn't require internal workarounds
- @preconcurrency tells compiler Crypto module hasn't adopted strict concurrency yet
- Will automatically work when swift-crypto adds proper Sendable conformance
- All 35 tests passing

Author: coenttb

Sources/JWT/RFC 7519+Crypto.swift

@@ -7,7 +7,7 @@
 
 import Foundation
 import RFC_7519
-import Crypto
+@preconcurrency import Crypto
 
 // MARK: - JWT Creation with swift-crypto