Bind @BeanParam using a custom builder such as immutables.io

Our project uses immutables.io to for its models. For methods that take several parameters, we wanted to box these parameters into a request object to improve readability and testing.

For consistency it would be nice to continue using immutables with these request objects. However, I could not find a way to customise `@BindParam` to support builders.

## Example

```java
public class Controller {
@GET @Path("items/{foo}")
public Response get(@BeanParam ItemRequest request) {
...
}
}

@Value.Immutable
public interface ItemRequest {
@PathParam("foo") String foo();
@QueryParam("bar") String bar();
}
```

## Workaround
It is possible to bind an immutable class using a builder if you create it yourself by using a second level of indirection on the `@BeanParam`. The downside is you have to write all the boilerplate instead of letting immutables generate it for you.

```java
public class Controller {
@GET @Path("items/{foo}")
public Response get(@BeanParam ItemRequest request) {
...
}
}

public class ItemRequest {
private final String foo;
private final String bar;

public ItemRequest(@BeanParam Builder builder) {
this.foo = builder.foo;
this.bar = builder.bar;
}

public static class Builder {
private String foo;
private String bar;

@PathParam("foo")
public Builder foo(final String foo) {
this.foo = foo;
return this;
}

@QueryParam("bar")
public Builder bar(final String bar) {
this.bar = bar;
return this;
}

public ItemRequest build() {
return new ItemRequest(this);
}
}
}
```