staging: add support for storage masking
This MR (finally!) introduces storage masking support for staging.
- Add support for the masking admin cli in the toolbox
- Add support for pre-processing pipeline steps definitions (needed to be able to generate the masking_db parameter out of a secret/envvar for the masking proxy)
- Use the masking proxy in both read-only storage backends
The postgresql read-only RPC backend had the retry pipeline step enabled; I think that was a mistake? So I split the readonly pipeline steps into two variables:
- readOnlyStoragePipelineSteps for the rpc clients
- readOnlyStorageBackendPipelineSteps for the rpc servers