support multi-subject attestations

Signed-off-by: Brian DeHamer <bdehamer@github.com>

packages/attest/__tests__/attest.test.ts

@@ -0,0 +1,16 @@
+import {attest} from '../src/attest'
+
+describe('attest', () => {
+  describe('when no subject information is provided', () => {
+    it('throws an error', async () => {
+      const options = {
+        predicateType: 'foo',
+        predicate: {bar: 'baz'},
+        token: 'token'
+      }
+      expect(attest(options)).rejects.toThrowError(
+        'Must provide either subjectName and subjectDigest or subjects'
+      )
+    })
+  })
+})