Green: unseal shared collection keys with the account keypair

decryptCollection now takes the full key material {masterKey,
publicKey, secretKey} and dispatches on keyDecryptionNonce: present
means an owned collection (secretbox under the master key), absent
means a shared collection (sealed box to our public key). Client
already held the keypair for unsealing the auth token, so it just
passes it through.

src/index.ts

@@ -24,6 +24,7 @@ export type {
     FileBlob,
     FileMetadata,
     FileType,
+    KeyMaterial,
     Microseconds,
     RawCollection,
     RawEnteFile,