Provide a common package to replace Pydantic models

[khvn26]

Rationale

Since flagsmith-engine does not export the Pydantic models representing Flagsmith entities anymore, we need a common package to avoid manually synchronising the schemas across their consumers (currently, Core and Edge API).

As part of this work, we are decoupling the internal DynamoDB schemas from the publicly available Environment Document API schema and removing the Pydantic dependency as a strict requirement for consuming them.

Requirements

Provide a Python package that includes the following:

• A set of types representing DynamoDB-stored Flagsmith documents.
• No third-party dependencies, except for type_extensions. This is to enable maximum portability for edge runtimes.

The package should NOT include mapping and validation logic.
The types should be usable by Pydantic type adapters where possible (i.e., the Core API).
The package should be installable independently of each other as Python package extras.

Use cases

Environment Documents

The DynamoDB schema is identical to the EnvironmentModel Pydantic class.

Operation	Target/source	Consumer
Write	DynamoDB	Flagsmith/flagsmith
Read	DynamoDB	Flagsmith/edge-api

Environment API key Documents

Operation	Target/source	Consumer
Write	DynamoDB	Flagsmith/flagsmith
Read	DynamoDB	Flagsmith/flagsmith
Read	DynamoDB	Flagsmith/edge-api

Identities

Operation	Target/source	Consumer
Write	DynamoDB	Flagsmith/flagsmith
Write	DynamoDB	Flagsmith/edge-api
Read	DynamoDB	Flagsmith/flagsmith
Read	DynamoDB	Flagsmith/edge-api