Add `atlas_events()` function

[mjwestgate]

The ALA events systems supports storage and download of event-core datasets, useful for storing data from field surveys or biodiversity monitoring programs (among other things). APIs for the ALA events system have been in production for some time, you can get basic info here and discussion of the graphQL schema here. At some point we should aim to build this into galah, with following syntax:

galah_call(type = "events") |>
  filter(year == 2020) |>
  collect()

or equivalently:

galah_call() |>
  filter(year == 2020) |>
  atlas_events()