add support for mixed json parsing

for example,

```scala
case class Foo(
  hello: String,
  @JsonParse
  world: List[Int]
)
```

Normally, you would need to submit a payload like this to match `Foo`
```
hello=something
world[0]=198
world[1]=897123
world[2]=9
```

We want to allow:
```
hello=something
world={198, 897123, 9}
```

It seems a `@JsonParse` annotation or something like that could do the trick