feat(core): refactor basemodels moving them to core

[marcos-sinch]

Refactor base models into core

Consolidates the duplicated per-domain BaseModelConfiguration classes (SMS,
Numbers, Number Lookup, Conversation) into three shared base classes under
sinch.core.models.internal.

• BaseConfigModel, SnakeCaseExtrasModel (camelCase→snake_case
  extras on validation), and CamelCaseDumpModel (snake_case→camelCase on dump
  when by_alias=True); each domain now aliases to the appropriate base.
• The Numbers model_dump override is reimplemented as a Pydantic v2 wrap
  serializer, so the camelCase rewrite now applies to model_dump_json too.
• No public behaviour change: all request serializers call model_dump(by_alias=True).
• Tests for the case helpers, each base class, and composition behaviour.