make ed25519 to use import flow by default

[himanshuchawla009]

• So far we generate ed25519 mpc keys using dkg but that makes it impossible to export the seed.
• To tackle this issue this PR changes the default ed25519 login flow to use import key flow such that seed is generated on frontend and thn same seed can be exported.
• Applications who still want to use dkg for ed25519 with the downside of key exportablity can still use it by setting useDkg flag to false in constructor explicitlu.

[matthiasgeihs]

made some comments. much of it is code style and i'll add a commit to clean that up.

note my other comment on the default value of the new useDKG option.

[matthiasgeihs]

just a minor comment on usage of scalarBNToBufferSEC1, see above. otherwise looks good i think