Support `object` JsonAdapter in `@AdapterBy`

[yogurtearl]

Support object adapters which would allow for creating fewer instances of the adapter.

@AdaptedBy(FooAdapter::class)
data class Foo(
     val bar: Bar
)

object FooAdapter: JsonAdapter<Foo> {
      override fun fromJson(reader: JsonReader): Foo { ... }
      override fun toJson(writer: JsonWriter, value: Foo) { ... }
}

Currently this gives an error of:

java.lang.IllegalAccessException: class dev.zacsweers.moshix.adapters.AdaptedBy$Factory 
                       cannot access a member of class com.example.FooAdapter with modifiers "private"

[ZacSweers]

PR welcome 👍