WIP: DiscoveryEngineDataStoreTargetSite: improve export functionality

[justinsb]

• DiscoveryEngineTargetSite: initial types
• DiscoveryEngineTargetSite: generated apimachinery
• DiscoveryEngineDataStoreTargetSite: create test
• tests: golden output
• Fixes to API (should go into previous PR)
• Autogen fixes (should go into previous PR)
• tests: normalization for DiscoveryEngineDataStoreTargetSite
• DiscoveryStoreDataStoreTargetSite: controller
• chore: create shared project number/id mapper
• DiscoveryEngineDataStoreTargetSite: mockgcp support
• DiscoveryEngineDataStoreTargetSite: golden files for export
• DiscoveryEngineDataStoreTargetSite: improve export functionality

[google-oss-prow]

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