Allow Undertow's server and socket options to be configured via the environment

Something like:

```
server.undertow.options.socket.whatever: foo
server.undertow.options.server.whatever: bar
```

`whatever` is then coerced into one of the constants on `UndertowOptions`, ideally with relaxed binding. A concrete example:

```
server.undertow.options.server.enable-http2: true
```

See #3350 which this issue probably supersedes.
