Support external-account-required metadata

acme4j-client/src/main/java/org/shredzone/acme4j/Metadata.java

@@ -65,6 +65,13 @@ public class Metadata {
                 .collect(toList());
     }
 
+    /**
+     * Returns whether an external account is required by this CA.
+     */
+    public boolean isExternalAccountRequired() {
+        return meta.get("external-account-required").orElse(false).asBoolean();
+    }
+
     /**
      * Returns the JSON representation of the metadata. This is useful for reading
      * proprietary metadata properties.

acme4j-client/src/test/java/org/shredzone/acme4j/SessionTest.java

@@ -249,6 +249,7 @@ public class SessionTest {
         assertThat(meta.getTermsOfService(), is(URI.create("https://example.com/acme/terms")));
         assertThat(meta.getWebsite(), is(url("https://www.example.com/")));
         assertThat(meta.getCaaIdentities(), containsInAnyOrder("example.com"));
+        assertThat(meta.isExternalAccountRequired(), is(true));
         assertThat(meta.getJSON(), is(notNullValue()));
     }
 

acme4j-client/src/test/resources/json/directory.json

@@ -9,6 +9,7 @@
     "caa-identities": [
       "example.com"
     ],
+    "external-account-required": true,
     "x-test-string": "foobar",
     "x-test-uri": "https://www.example.org",
     "x-test-array": [