feat: upgrade OpenMLS 0.5 → 0.8 for security patches and GREASE support

Migrates all MLS code in quicprochat-core from OpenMLS 0.5 to 0.8:
- StorageProvider replaces OpenMlsKeyStore (keystore.rs full rewrite)
- HybridCryptoProvider updated for new OpenMlsProvider trait
- Group operations updated for new API signatures
- MLS state persistence via MemoryStorage serialization
- tls_codec 0.3 → 0.4, openmls_traits/rust_crypto 0.2 → 0.5

crates/quicprochat-core/Cargo.toml

@@ -15,6 +15,7 @@ native = [
     "dep:openmls",
     "dep:openmls_rust_crypto",
     "dep:openmls_traits",
+    "dep:openmls_memory_storage",
     "dep:tls_codec",
     "dep:opaque-ke",
     "dep:bincode",
@@ -49,6 +50,7 @@ opaque-ke           = { workspace = true, optional = true }
 openmls             = { workspace = true, optional = true }
 openmls_rust_crypto = { workspace = true, optional = true }
 openmls_traits      = { workspace = true, optional = true }
+openmls_memory_storage = { workspace = true, optional = true }
 tls_codec           = { workspace = true, optional = true }
 bincode             = { workspace = true, optional = true }