feat: [Firestore] expose the variable definition in the Cloud Firestore API (#8958)

* feat: expose the variable definition in the Cloud Firestore API
chore: Firestore.executePipeline to not retry on `RESOURCE_EXHAUSTED`

PiperOrigin-RevId: 877535984

Source-Link: googleapis/googleapis@1ccd68a

Source-Link: googleapis/googleapis-gen@eaf17cf
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Firestore/metadata/V1/Document.php

@@ -8,7 +8,6 @@
                     "UNAVAILABLE"
                 ],
                 "retry_policy_2_codes": [
-                    "RESOURCE_EXHAUSTED",
                     "UNAVAILABLE",
                     "INTERNAL",
                     "DEADLINE_EXCEEDED"