Add GPL-3.0 LICENSE to flepimop2-op_engine provider package

[jc-macdonald]

Add an explicit GPL-3.0 license file to the flepimop2-op_engine/ provider package directory and declare it in the provider's pyproject.toml.

Scope

• Add flepimop2-op_engine/LICENSE with GPL-3.0 text
• Add license = { file = "LICENSE" } to flepimop2-op_engine/pyproject.toml
• Add GPL-3.0 classifier to provider pyproject.toml

Context

The provider package connects to flepimop2 which is GPL-3.0, so this connector must remain GPL-3.0 even as the core engine moves to MIT (#34).