Allow web socket connections to refresh their token

[ctron]

Merging PR #290 periodically checks the token validity. However, this also means that the user may get a way to provide a refreshed/new token.

• [x] https://github.com/drogue-iot/drogue-cloud/issues/295
• [x] https://github.com/drogue-iot/drogue-cloud/issues/296
• [x] https://github.com/drogue-iot/drg/issues/140