Add curve 25519 support

[dlubawy]

Yubico added support for curve 25519 in the PIV applet since firmware '5.7.X'. This support has already been added to yubico-piv-tool, but it is lacking in this Rust library. This PR is an attempt to keep the libraries aligned on functionality. Changes made are as described:

• curve25519-dalek crates added as dependencies
• X25519 and Ed25519 added as available algorithms
• Updated the integration tests to include an ED25519 signature check
  • Tests did not seem to work well for me (constant poisoned mutex issue) so I made some modifications but can revert these if necessary
  • Wise to double check these work as intended

I'm open to any feedback. This is my first real go at production Rust code so criticism is welcome/needed for me to learn.

Additional items to maybe consider:

• I'm not sure how this will impact anyone using firmware older than '5.7.X'
  • Might want to add a check on that and throw NotSupported errors
  • Otherwise, take an approach where firmware version is tied to release version?