feat: add client data model

[cdavernas]

This pull request introduces a new ClientDataModel class to the core library, updates serialization support, and adds corresponding unit tests. It also refines the immutability of the Version property in existing records and increments the project version. The most important changes are grouped below:

New Client Data Model Implementation:

• Added the ClientDataModel record to represent client-maintained object data models, including versioning and surface state mapping. (src/A2UI.Core/ClientDataModel.cs)
• Registered ClientDataModel for JSON serialization in JsonSerializationContext. (src/A2UI.Core/Serialization/Json/JsonSerializationContext.cs)

Testing Support:

• Introduced unit tests for serialization/deserialization of ClientDataModel in ClientDataModelTests. (tests/A2UI.UnitTests/Cases/Core/ClientDataModelTests.cs)
• Added a factory class for generating sample ClientDataModel instances in tests. (tests/A2UI.UnitTests/Services/ClientDataModelFactory.cs)

API Consistency and Versioning:

• Changed the Version property in ClientSideEvent and Message records from set to init to enforce immutability. (src/A2UI.Core/ClientSideEvent.cs, src/A2UI.Core/Message.cs) [1] [2]
• Incremented the project version prefix from 0.2.0 to 0.3.0 in A2UI.Core.csproj. (src/A2UI.Core/A2UI.Core.csproj)