verify: Don't assume signature method, use key type
verify: Don't assume signature method, use key type

It is unsafe to use the signature method field, as it could result
in a key confusion attack. Use the key type instead.

Also, don't assume that we're only working with Ed25519.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

theupdateframework/go-tuf