Add UUID path extractor

[tmfset]

Add UUIDVar, a path extractor for java.util.UUID

I also considered adding a ValidatedUUIDVar to extract a Validated[Throwable, UUID] from paths but then realized that there is already a simple way to return BadRequest for invalid UUIDs:

HttpService[IO] match {
  case GET -> Root / "user" / UUIDVar(userId) => ...
  case GET -> Root / "user" / str => BadRequest(s"Invalid identifier '$str'")
}

The original PR targeted 0.18.x but broke binary compatibility, this new one targets master to avoid that complication.

[rossabaker]

👍

[ChristopherDavenport]

👍 Going to have merge conflicts on the merge train with the backwards compatible stuff.