fix: send_device_attestation encoding of attestation objects

[christianhoelzl]

This pull request fixes the function send_device_attestation. Any binary (CBOR encoded) attestation object DeviceAttestation::att_obj is now sent as an base64 encoded string to the ACME CA, not as an array anymore. I successfully tested this change with Step CA. Some obsolete annotations are removed from the public DeviceAttestation struct.

[djc]

Thanks! Is this from a lack of testing before or has the spec changed? (Same question for your other PR.)

[christianhoelzl]

This was a lack of testing with a real CA, not a change in the ACME specs.
My private ACME test server did accept an array and the bug was overlooked.

[cpu]

@djc OK to merge this & #254 now or did you want to wait to hear back about Pebble support?