Make FreePortFinder support Jackson's polymorphic deserialization

[sleberknight]

FreePortFinder should allow for deserialization into a specific type using Jackson's polymorphic deserialization feature.

This will allow new implementations to be easily configured in YAML. For example:

dynamicPorts:
  freePortFinder:
    type: lowHigh
  minDynamicPort: 27000
  maxDynamicPort: 29000
  tls:
    keyStorePath: /data/etc/pki/my-ks.jks
    keyStorePassword: my-ks-pwd
    trustStorePath: /data/etc/pki/my-ts.jks
    trustStorePassword: my-ts-pwd

In the above the freePortFinder is configured using the type property to specify the implementation, which needs to map to a class annotated with the same type in a @JsonTypeName annotation. For example:

@JsonTypeName("lowHigh")
public static class LowHighFreePortFinder implements FreePortFinder {
    // implementation details omitted...
}

For the three out-of-the-box implementations, use the following as the type name:

Implementation Class	Type Name
AdjacentFreePortFinder	adjacent
IncrementingFreePortFinder	incrementing
RandomFreePortFinder	random

All this "just works" because of Dropwizard support for Polymorphic configuration