Add support for elliptic curve/ed25519 SSH keys

Adds a dependency on net.ip2.crypto.eddsa, which is released under
the CC0 1.0 Universal license [1].

[1] https://github.com/str4d/ed25519-java/blob/master/LICENSE.txt

Feature: Issue 4507
Change-Id: Icab3a3e367b69c14132f0cc9478a84ebb9df834c

lib/mina/BUILD

@@ -2,10 +2,22 @@ java_library(
     name = "sshd",
     data = ["//lib:LICENSE-Apache2.0"],
     visibility = ["//visibility:public"],
-    exports = ["@sshd//jar"],
+    exports = [
+      ":eddsa",
+      "@sshd//jar"
+    ],
     runtime_deps = [":core"],
 )
 
+java_library(
+    name = "eddsa",
+    data = ["//lib:LICENSE-CC0-1.0"],
+    visibility = ["//visibility:public"],
+    exports = [
+        "@eddsa//jar",
+    ],
+)
+
 java_library(
     name = "core",
     data = ["//lib:LICENSE-Apache2.0"],