Integrate with lifecycle package

[schloerke]

Remaining questions:

• Should plumber be transformed to Plumber?
  • Should hookable be transformed to Hookable?
  • If so, should plumber still exist?
• Should plumber's historical R6 methods be converted to snake_case?
  • If so, should PlumberEndpoint's R6 methods be converted to snake_case?

---

• plumber$run() args now use lifecycle package and deprecation notice
• randomCookieKey() -> random_cookie_key()
• sessionCookie() -> session_cookie()
• getCharacterSet() -> get_character_set()

PR task list:

• Update NEWS
• Add tests
• Update documentation with devtools::document()