getRkpData

Returns the parsed, but generic contents of the [Remote Key Provisioning

extension](https://source.android.com/docs/security/features/keystore/attestation#provisioninginfo_extension), if present in an Android attestation certificate chain. One would assume that we could define a type-safe data structure for that, but Samsung being Samsung has kindly reminded us of the fact that phrases like "conforms schema" are thrown around far too often in specifications.

Google's code has such a type for that, but I wouldn't trust vendors to observe the CBOR schema, so we just check for valid CBOR as a baseline.