AWS RDS Data API (rds-data) restJson1 service for fakecloud.
Batch 1: vendored Smithy model + crate scaffold + restJson1 routing for the
Data API operations + a real ExecuteStatement that resolves the
resourceArn to a fakecloud-managed RDS container DB and runs the SQL for
real over tokio-postgres / mysql_async, returning AWS-shaped typed
Field records + columnMetadata. This is the wedge: rival emulators serve
canned rows or return {} for binary columns; fakecloud runs genuine SQL on
a real engine. Transactions + BatchExecuteStatement land in later batches;
unimplemented operations return a faithful BadRequestException/not-yet
error rather than a fake success.