Add API call for workflow and inputs validation

[markjschreiber]

Many workflow engines can validate that a workflow is syntactically correct and inputs/ input files are correct. Making this capability available via an API call would be very valuable.

The response could be 200 OK with a body including a list of errors, or an empty list if there are no errors. Alternatively a 203 No Content response might be used to indicate no error?

More capable engines may even be able to perform a dry run as part of the checks.

[uniqueg]

I like the idea, but I would expect that this is still quite a long/blocking operation, at least for some engines, and so it might need to be handled asynchronously. In that case, a 200 or 203 response would not be sufficient, but rather this would need to be implemented similar to a regular run, an identifier would need to be returned, a status attached etc.

An alternative then to adding one (or possibly more!) endpoints for validation/dry runs, would be just passing the appropriate parameters to workflow-engine-parameters, like, e.g., in Nextflow and Snakemke with -stub-run and --dry-run, respectively. And the ability to return multiple error responses (great idea!) might potentially be implemented through modifications of the ErrorResponse schema.

[patmagee]

@markjschreiber this has come up a number of times. I have created a broader ticket to discuss this: #176