Expose the functionality to decrypt given data using keyring

This allows us to provide the keyring a blob of encrypted data and a key name, and have it decrypt it for us.

Also fixed javadoc length in Keyring.java. It seems like it was using a 80-character length limit.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222995542

java/google/registry/keyring/api/KeyModule.java

@@ -20,6 +20,7 @@ import dagger.Module;
 import dagger.Provides;
 import java.lang.annotation.Documented;
 import java.util.Optional;
+import java.util.function.BiFunction;
 import java.util.function.Function;
 import javax.inject.Named;
 import javax.inject.Qualifier;
@@ -134,4 +135,10 @@ public final class KeyModule {
   static Function<String, String> provideEncryptedDataRetriever(Keyring keyring) {
     return keyring::getEncryptedData;
   }
+
+  @Provides
+  @Named("keyringDecrypter")
+  static BiFunction<String, String, byte[]> provideKeyringDecrypter(Keyring keyring) {
+    return keyring::getDecryptedData;
+  }
 }