Use custom serializer for ScientificUnits instead of generated

[vpodlesnyak]

Is your feature request related to a problem? Please describe.
The generated serializer uses a full class name for serialization and in case of kotlin-firebase-sdk a class

package dev.gitlive.firebase

@Serializable
sealed class SealedClass {
    @Serializable
    object AnObject: SealedClass()
}

is serialized as

[dev.gitlive.firebase.SealedClass.AnObject, {}]

which

• is redundant
• prevents the sealed class ever to be moved to a different package
• creates complications when stored with Firestore in some cases - nested arrays are forbidden

Describe the solution you'd like
A suggested solution would be to create a custom serializer or a set of serializers which would encode Scientific units as a plain string