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/files/nomulus-config-unittest.yaml

@@ -27,3 +27,7 @@ caching:
 # tests
 gSuite:
   supportGroupEmailAddress:
+
+misc:
+  # We would rather have failures than timeouts, so reduce the number of retries
+  transientFailureRetries: 3