feature request: support retry with backoff

[fuyufjh]

RisingWave takes use of this project to test everything via SQL interface.

Some test cases involve asynchronous operations, which makes them inherently falky. For example,

• A test that reads data from Kafka source. There is no exact upper bound to ensure the data is consumed. Example
• A test that relies on an alter system set command, where in RisingWave these changes are propagated to compute/frontend nodes in background. Example

Proposal

Shall we introduce something like

[statement [error|ok] | query] retry 3 backoff 5s

Which means the case will be retried for 3 times, with waiting 5 second before each retry.