Add composed option to datasources

[mingard]

Add a composed property to the datasource schema to override collection defaults.

[jimlambie]

@mingard this could be done by adding ?compose=false to the endpoint within the source block

[mingard]

@jimlambie It would be useful to have this as a parameter in the DS similar to filters, count and fields.

[mingard]

If you don't see it as useful by all means close the ticket. I was intending on getting round to it...eventually!