[Extensions] Introduce more informative enum for Extension results

[msirringhaus]

From my reading of the spec, the result of client extension processing on either get() or create() could be:

1. Accept - Use the client extension input as client extension output. This is used for authenticator extensions that require no processing on the client, eg. 12.3. Large Blob Key (largeBlobKey): Client extension input / output / processing.
2. Accept & update - The client re-writes the authenticator extension input, with different fields, e.g. 12.5 HMAC Secret Extension (hmac-secret): Client extension processing.
3. Drop - Remove this extension's payload altogether from the authenticator request, e.g. 12.1. Credential Protection (credProtect): Client extension processing drops the create() payload if credBlob size is unexpected.

enum ClientExtensionProcessingResult {
  Accept(cbor::Map),
  Drop,
}  

Originally posted by @AlfioEmanueleFresta in #77 (comment)