fix: use MetadataAuditStamp instead of com.linkedin.common.AuditStamp

[jsdonn]

Saw some downstream errors complaining about namespace mismatch i.e.

com.linkedin.schemaregistry.validation.IncompatibleSchemaException: com.linkedin.schemaregistry.AvroSchemaRenameException: (suspected) attempt to rename nested RECORD com.linkedin.common.AuditStamp to com.linkedin.pegasus2avro.common.AuditStamp
At :MetadataAuditEvent:auditStamp:union

The reason is because the downstream (metadata-models) explicitly excludes core-models module, which com.linkedin.common.AuditStamp is a part of. So one solution to this is to define our own AuditStamp model and use that instead. To avoid confusion, renamed to MetadataAuditStamp.

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