feat(webauthn): LargeBlobStorage trait + in-memory + authenticator backends

Adds the public `LargeBlobStorage` async trait alongside two bundled
implementations:

- `MemoryLargeBlobStorage`: a HashMap-backed store, primarily for tests.
- `AuthenticatorLargeBlobStorage<'_, C>`: drives the CTAP 2.1
  `authenticatorLargeBlobs(get)` command, parses the serialized
  largeBlobArray, locates the entry matching the supplied per-credential
  `largeBlobKey` (AES-256-GCM authenticated decryption), and decompresses
  the deflated plaintext.

Only the read path is implemented in this PR. `LargeBlobStorage::write`
returns `Unsupported` in both bundled backends; the chunked write path
with `pinUvAuthParam` binding is reserved for a follow-up.

Includes 14 unit tests covering: in-memory round-trip, AEAD round-trip,
wrong-key rejection, multi-entry array selection, corrupted/truncated
array rejection, empty array handling, and a MockChannel-backed end-to-end
test of the authenticator read flow.

Refs: WebAuthn L3 §10.5, CTAP 2.2 §6.10 / §6.10.4 / §11.4.

Author: AlfioEmanueleFresta

libwebauthn/Cargo.lock

@@ -58,6 +58,8 @@ p256 = { version = "0.13.2", features = ["ecdh", "arithmetic", "serde"] }
 heapless = "0.7"
 cosey = "0.3.2"
 aes = "0.8.2"
+aes-gcm = "0.10"
+flate2 = "1.0"
 hmac = "0.12.1"
 cbc = { version = "0.1", features = ["alloc"] }
 hkdf = "0.12"