Improve `Endpoint` panic-safety and ergonomics (#1984)

- `Endpoint::set_endpoint` no longer panics when called on an endpoint
  without a scheme
- `Endpoint::mutable` and `Endpoint::immutable` now both return a result
  so that constructing an endpoint without a scheme is an error
- `Endpoint::mutable` and `Endpoint::immutable` both now take a string
  instead of a `Uri` as a convenience
- `Endpoint::mutable_uri` and `Endpoint::immutable_uri` were added
  to construct an endpoint directly from a `Uri`