TRITON-2417 Add support for ED25519 keys

Testing Notes

Confirmed that make check succeeded.
Installed changes via sdcadm up -C experimental cloudapi@TRITON-2378-20231206T170927Z-gee2e002
Ran the integration tests on my local headnode and confirmed that there are no new failures (there are a few unrelated failures that were failing before these changes and will be fixed separately e.g. TRITON-2415, TRITON-2416)
Updated sshpk in sdc-ufds and confirmed that make check succeeded and that I could install the changes via sdcadm update -C experimental ufds@TRITON-2378-20231116T205537Z-g5fb2ee0
I then updated sskpk-agent and smartdc-auth to latest in node-triton, removed all keys from my account except for an ED25519 key and confirmed that I could make Cloud API requests, provision an instance, and ssh into it (provided a CN with an image containing OS-8502 was used.)
I also used smartdc-auth to generate a signed CloudAPI URL and confirmed the URL was authenticated and returned the expected response.

Worth noting that these changes have also been used while testing the following PRs:

TritonDataCenter / sdc-cloudapi