set user agent

[sbfnk]

Fixes #70

Seems to work

GET(
    "http://httpbin.org/user-agent",
    config = config(
      followlocation = 1,
      useragent = paste0(
        "https://github.com/epiforecasts/socialmixr R package socialmixr/v.",
        packageVersion("socialmixr")
      )
    )
  )
Response [http://httpbin.org/user-agent]
  Date: 2023-07-27 09:03
  Status: 200
  Content-Type: application/json
  Size: 98 B
{
  "user-agent": "https://github.com/epiforecasts/socialmixr R package socialmi...
}