Support ECDSAP256SHA256 with real KMIP servers.

[ximon18]

See NLnetLabs/domain-kmip#5. We don't have this issue with Cascade-HSM-Bridge as Cascade and Cascade-HSM-Bridge each know what the other expects, but with a real KMIP HSM that we connect to directly instead of connecting to the bridge (which if for PKCS#11 HSMs) it isn't currently possible (at least with Securosys Cloud HSM, and maybe/probably? with other HSMs too).

It's unclear to me if https://cascade.docs.nlnetlabs.nl/en/latest/hsms.html is incorrect in stating support for Fortanix DSM via KMIP. I did test with Fortanix DSM, but am unsure if it definitely worked with ECDSAP256SHA256 or only with RSA, and the free trial I used has since expired.

[ximon18]

Status update: I managed to get my locally modified Cascade to sign with ECDSAP256SHA256 using a Securosys cloud KMIP server. It actually only required a one line change! (plus all of the changes needed for #982 of course).