Refactor common email sending utility

The main thrust of this is to create a common POJO that contains email content in a simple way, then have one class that converts that to an email and sends it. Any class that uses email should only have to deal with creating that POJO.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237883643

java/google/registry/config/RegistryConfigSettings.java

@@ -169,6 +169,7 @@ public class RegistryConfigSettings {
     public String alertRecipientEmailAddress;
     public String spec11ReplyToEmailAddress;
     public int asyncDeleteDelaySeconds;
+    public int transientFailureRetries;
   }
 
   /** Configuration for keyrings (used to store secrets outside of source). */