Akka-http: Add Java API to create custom MediaTypes

[jlprat]

As discussed in https://discuss.lightbend.com/t/akka-http-java-api-need-to-create-a-contenttype-from-a-string-mime-type/957 we discovered that one of the Scala APIs is missing for the Java one.

private val custom: MediaType.Binary = MediaType.customBinary("application", "wat", MediaType.Compressible)
private val binary = ContentType(custom)
println(s"binary = ${binary}")

To ease Java API users, we should have the same APIs available for both languages.

[jlprat]

Closed via https://github.com/akka/akka-http/commit/f9622687075f731f047c36c0a62297557af7ef1a

[ktoso]

Thanks :)