Use offset for query result pagination

[hongyuanjia]

Currently, init_cmip6_index() only returns the first 10,000 records.

epwshiftr/R/esgf.R

Lines 446 to 452 in 09827d4

	if (nrow(qd) == 10000L) {
	warning("The dataset query returns 10,000 results which ",
	"hits the maximum record limitation of a single query using ESGF search RESTful API. ",
	"It is possible that the returned Dataset query responses are not complete. ",
	"It is suggested to examine and refine your query."
	)
	}

It is enough for most use cases. But it will still be good to implement pagination.