cred: support for enterprise attestation

[LDVG]

In short, the following user visible changes are applied:

• fido_cred_set_entattest() is added for the client to be able to request enterprise attestation;
• fido_cred_entattest() is added to query whether the authenticator performed enterprise attestation; and
• fido2-cred -M -a is added to request enterprise attestation (dito examples/cred).

Internally,

• fido_dev_make_cred_tx() learned how to encode the enterprise attestation parameter;
• fido_dev_make_cred_rx() learned how to decode the response; and
• likewise for the winhello.c equivalents.

Finally, the fuzzer and manual pages are updated accordingly.

Further resources:

• https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html#sctn-feature-descriptions-enterp-attstn
• https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html#op-makecred-step-if-enterpriseAttestation