Implement migration identity keys

[nzagorchev]

What	Where/Who
JIRA Issue	SDK-2684
People Involved	@nzagorchev

Background

Identity keys are received through getMigrateState and set in the CleverTap configuration.
Example JSON:

{
    "ct" : {
        "accountId" : "accId",
        "attributeMappings" : ...,
        "identityKeys" : ["Identity", "Phone", "Email"]
    };
    ...
}

The default value is "Identity".

If the keys are missing in the API response, or they are an empty array, they will not be updated. This means that previously saved or default keys will be used.
The keys that come from the API will not be further validated and will be passed directly as-is to the CT Config.

Implementation

• Add the identityKeys to the MigrationManager
• Refactor the response handling to use Codable
• Add unit tests

Testing steps

Is this change backwards-compatible?