Addition of checkonly flag to validateAgainstOrg deploy

[rrgripp]

I've seen that the validateAgainstOrg converts to MDAPI format and deploys it. The mdapi deploy command has a checkonly option so it doesn't persist the changes into the environment. (https://developer.salesforce.com/docs/atlas.en-us.sfdx_cli_reference.meta/sfdx_cli_reference/cli_reference_force_mdapi.htm#cli_reference_force_mdapi_deploy)

Would be interesting if we could use this flag (could be optional) in order to validate against the actual production environment without persisting this metadata.

The current scenario is that my pipeline validates against an staging org before publishing the package in production, but we added a job to deploy metadata to staging on merge requests (for more complete user testing and QA) and since, every time I deploy to staging, my validate against staging fails because No changes detected in the packages to be built, validate will only execute if there is a change in atleast one of the packages. With this flag I would be able to validate against staging and deploy metadata to it during the merge request pipeline and then, on the main branch pipeline, I would validate against production before installing the package there.

Maybe I'm missing something, so I'm open to other suggestions on how to approach this scenario. Thanks!