Convert observational data functions from httr to httr2

[Aariq]

The first piece of work transitioning from httr to httr2. This creates a base_req in zzz.R that is then used in all the modified functions to form the basic request. It includes only the API URL and a user agent for now, but could include rate limiting, retry, etc.

Most functions are updated in the same way—instead of npn_GET(<endpoint>, <query args>, ...) they are now formed like:

#form the request
req <- base_req %>%
    httr2::req_url_path_append("endpoint.json") %>%
    httr2::req_url_query(arg = arg)
#perform the request
resp <- httr2::req_perform(req)
#get the results as a data frame
out <- httr2::resp_body_json(resp)
#return as a tibble
tibble::as_tibble(out)

Possible breaking changes include:

• Several functions return tibbles now instead of data.frames. Tibbles are also data.frames, so this usually doesn't cause a problem. Where it might cause issues for users is when functions previously returned NULL (e.g. if there was an error) they now return a 0x0 tibble. I personally think this is an improvement, but we could keep the old behavior.
• The ... argument to functions no longer does anything (more in What to do about ... in npn_GET()? #46)
• npn_get_phenophases_for_taxon() now returns a tibble instead of a list (the documentation said it returned a data frame so I assumed this was a bug)
• npn_species_state() now errors instead of returningNULL when arguments are invalid. It sounded like this is the intended behavior from the documentation, but could mess up people's workflows