Add script to convert datastore-indexes.xml to index.yaml

The gcloud datastore cleanup-indexes command takes its input in index.yaml form instead of datastore-indexes.yaml form. This simple translation script allows us to generate one from the other.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=169260811

python/google/registry/scripts/testdata/index.yaml

@@ -0,0 +1,93 @@
+indexes:
+
+- kind: ContactResource
+  properties:
+  - name: currentSponsorClientId
+  - name: deletionTime
+
+- kind: DomainBase
+  properties:
+  - name: ^i
+  - name: currentSponsorClientId
+  - name: deletionTime
+
+- kind: DomainBase
+  properties:
+  - name: ^i
+  - name: tld
+  - name: deletionTime
+
+- kind: DomainBase
+  properties:
+  - name: currentSponsorClientId
+  - name: deletionTime
+
+- kind: HostResource
+  properties:
+  - name: currentSponsorClientId
+  - name: deletionTime
+
+- kind: RegistrarBillingEntry
+  ancestor: yes
+  properties:
+  - name: currency
+  - name: created
+    direction: desc
+
+- kind: DomainBase
+  properties:
+  - name: allContacts.contact
+  - name: deletionTime
+
+- kind: DomainBase
+  properties:
+  - name: nsHosts
+  - name: deletionTime
+
+- kind: DomainBase
+  properties:
+  - name: ^i
+  - name: nsHosts
+  - name: deletionTime
+
+- kind: HostResource
+  properties:
+  - name: inetAddresses
+  - name: deletionTime
+
+- kind: PollMessage
+  properties:
+  - name: clientId
+  - name: eventTime
+
+- kind: PollMessage
+  ancestor: yes
+  properties:
+  - name: clientId
+  - name: eventTime
+
+- kind: HistoryEntry
+  ancestor: yes
+  properties:
+  - name: modificationTime
+
+- kind: DomainBase
+  properties:
+  - name: ^i
+  - name: fullyQualifiedDomainName
+
+- kind: DomainBase
+  properties:
+  - name: ^i
+  - name: tld
+  - name: fullyQualifiedDomainName
+
+- kind: HostResource
+  properties:
+  - name: deletionTime
+  - name: fullyQualifiedHostName
+
+- kind: ContactResource
+  properties:
+  - name: deletionTime
+  - name: searchName