Implement refresh for Gargle2.0

[jennybc]

I don't think we can just inherit this from Token2.0 because removing a token from the cache is different for Gargle2.0 tokens. This is also not going to come up very often, i.e. is not urgent, but is worth doing.

relevant excerpt from httr::Token2.0:

  refresh = function() {
    cred <- refresh_oauth2.0(self$endpoint, self$app,
        self$credentials, self$params$user_params, self$params$use_basic_auth)
    if (is.null(cred)) {
      remove_cached_token(self)   # <-- this won't actually un-cache a Gargle2.0 token
    } else {
      self$credentials <- cred
      self$cache()
    }
    self
  },

[jennybc]

This is pretty difficult to fix since Token2.0's $refresh() method calls the unexported and very meaty function httr:::refresh_oauth2.0():

https://github.com/r-lib/httr/blob/af25ebd0e3b72d2dc6e1423242b94efc25bc97cc/R/oauth-refresh.R

I do not want to re-implement or copy that. The only thing I need to modify in the $refresh() method is the definition of remove_cached_token():

• To un-cache a Token2.0, you delete its component from the list of tokens that is the cache.
• To un-cache a Gargle2.0, you delete its file from the cache directory.

@hadley do you have any ideas?