check-resource: Trigger discovery of new versions

To force immediate checking for new versions of a resource, rather than waiting for the periodic checking, run:

$ fly -t example check-resource --resource my-pipeline/my-resource

To check from a particular version, including the given version, append the --from flag like so:

$ fly -t example check-resource --resource my-pipeline/my-resource \
    --from ref:abcdef

This can be useful for collecting versions that are older than the current ones, given that a newly configured resource will only start from the latest version.

Note the ref: prefix is resource-dependent. For example, the bosh-io-release resource might use version:11.2 in place of ref:abcdef.