Fix hierarchy of CommonDataEntity

[vestrel00]

While working on fixing #109, I noticed that there is no dedicated interface for immutable data.

For example, for Contacts the hierarchy is as follows,

ContactEntity
Contact : ContactEntity
MutableContact : ContactEntity

The MutableContact does NOT extend the immutable Contact. This allows a concrete distinction between mutable and immutable contacts. Consumers can do the following;

val contactEntity: ContactEntity
when (contactEntity) {
    is Contact -> handleImmutableContact()
    is MutableContact -> handleMutableContact()
}

However, for data, the hierarchy is as follows,

CommonDataEntity
MutableCommonDataEntity : CommonDataEntity

The MutableCommonDataEntity extends CommonDataEntity. We have no reliable way to know if a reference to a CommonDataEntity is actually immutable! This is prone to bugs such as #109 🤦

This needs to be fixed 🔥 🔥🔥🔥🔥🔥

[vestrel00]

This is a part of v0.1.9 Release Checklist.