Keystore: Wire up Ed25519 signing

Wire Ed25519 signing into Keystore. This consists of registering a
provider for Ed25519.

Ed25519 has its own digest scheme, so the caller should specify "none"
as the digest scheme, and that's the tag that's going to be passed into
KeyMint.
However, unlike other uses of the "NONE" digest scheme, the input to the
signature algorithm should not be truncated.

Bug: 194359292
Test: atest android.keystore.cts.Curve25519Test
Change-Id: Icce4f7f2f8fa10081a9c6beff4813c2d91756469