feat: add new types ExplainOptions, ExplainMetrics, PlanSummary, ExecutionStats (#1241)

* feat: add new types ExplainOptions, ExplainMetrics, PlanSummary, ExecutionStats
feat: add ExplainOptions field to RunQueryRequest
feat: add ExplainMetrics field to RunQueryResponse
feat: add ExplainOptions field to RunAggregationQueryRequest
feat: add ExplainMetrics field to RunAggregationQueryResponse

PiperOrigin-RevId: 615158168

Source-Link: https://github.com/googleapis/googleapis/commit/4d535ac0538bb2d4b406250d7ec10b25a17a54cf

Source-Link: https://github.com/googleapis/googleapis-gen/commit/02e272ded538b0f97832bfad47decbc3dc65a89a
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