Switch from httr to httr2

[pitkant]

• Changed all references to httr in code to httr2

TO DO:

• clarify in function documentation how to continue using proxy in the future. Current documentation gives the following advice:

#'   To use a proxy to connect, a [httr::use_proxy()] can be
#'   passed to [httr::GET()]. For example
#'   `get_eurostat_json(id, filters,
#'   config = httr::use_proxy(url, port, username, password))`.

[pitkant]

Proxy functionalities are now implemented, needs only testing