Add a Beam pipeline to generate RDE deposit (part 1) (#1219)

This is the first part of the RdeStagingAction SQL migration where the
mapper logic is implemented in Beam.

A few helper methods are added to convert the DomainContent, HostBase
and ContactBase to their respective terminal child classes. This is
necessary and possible because the child classes do not have extra
fields and the base classes exist only to be embedded to other entities
(such as the various HistoryEntry entities). The conversion is necessary
because most of our code expects the terminal classes, such as the
RdeMarshaller's various marshallXXX() methods. The alternative would be
to change all the call sites, which seems to be much more disruptive.

Unfortunately there is is no good way to do this conversion than just
creating a builder and setting every fields there is.

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release/cloudbuild-nomulus.yaml

@@ -93,7 +93,9 @@ steps:
       google.registry.beam.spec11.Spec11Pipeline \
       google/registry/beam/spec11_pipeline_metadata.json \
       google.registry.beam.invoicing.InvoicingPipeline \
-      google/registry/beam/invoicing_pipeline_metadata.json
+      google/registry/beam/invoicing_pipeline_metadata.json \
+      google.registry.beam.rde.RdePipeline \
+      google/registry/beam/rde_pipeline_metadata.json
 # Tentatively build and publish Cloud SQL schema jar here, before schema release
 # process is finalized. Also publish nomulus:core jars that are needed for
 # server/schema compatibility tests.