Allow a prepared ENA metadata samplesheet as input

[Midnighter]

Description of feature

I've created a branch https://github.com/nf-core/fetchngs/tree/skip-runinfo where one can skip the first step. So instead of providing a list of IDs for which metadata is fetched, provide the metadata directly as input and skip that step. It works but was just a quick hack. If this is a generally desirable feature I can code it up in a better way. Maybe using input_type == 'metadata' instead of a new skip_runinfo parameter.

[priyanka-surana]

It would be useful for our team too. 👍🏼 from me if you decide to implement this feature.