collections: allow a collection to be set/mapped from a spreadsheet

I want to:

• define a bunch of mappings in a spreadsheet (XLS or gsheets)
• Pull those mappings into the CLI
• Upload them into a collection

This feels like it's something the CLI is very capable of doing.

If the sheet just has two columns, we can just pull them as key/value pairs. Otherwise we need to take a range (or I suppose two columns). We may need to know whether the first two rows are a header and can be ignored (although it's probably harmless to include a header in the map - it's just one redundant key)

The mapping should be a single JSON object uploaded as a key to a single value

So the CLI command is like:

openfn collections set my-collection my-key --items pathOrUrlToSheet

Maybe we need extra CLI args for "parse items as spreadsheet" or "choose this range" or "ignore headers".

Start by reviewing the CLI's existing collection commands and the proposed collections set my-collection my-key --items pathOrUrlToSheet entry point. Clarify how XLS or Google Sheets input, ranges, and headers should be handled. Done means spreadsheet mappings are converted to one JSON object and uploaded as one collection value, with the command behavior defined.