Warning about embedding fields that should have encoders

[deusaquilus]

One significant problem occurs with auto-embedded changes introduced in #2607
In cases where you have a single-value case class inside an entity it will become embedded so it is confusing why the column of the outer entity will be replaced when the inner one is missing an encoder. For example:

case class MyIdType(value: String)
case class Person(id: MyIdType, name: String)
run(query[Person])
// SELECT x.value /*instead of x.id*/, x.name FROM Person x,

Since MyIdType is being treated as an embedded entity, it is automatically embedded and it's value field is being used as the column instead of id. Once the encoder for MyIdType is written this problem will go away. Need to warn the user in cases where the embedded entity has one field that this is likely a column that needs to be embedded.