Add Layer 2 Circuit product blocks and creation workflow

gso/products/product_blocks/layer_2_circuit.py

gso/products/product_blocks/service_binding_port.py

gso/products/product_types/layer_2_circuit.py

gso/services/lso_client.py

gso/utils/types/interfaces.py

gso/utils/types/virtual_identifiers.py

gso/utils/workflow_steps.py

gso/workflows/__init__.py

gso/workflows/l2_circuit/__init__.py

gso/workflows/l2_circuit/create_layer_2_circuit.py

...n_l3_core_service/create_imported_nren_l3_core_service.py

...flows/nren_l3_core_service/create_nren_l3_core_service.py

...lows/nren_l3_core_service/migrate_nren_l3_core_service.py

...flows/nren_l3_core_service/modify_nren_l3_core_service.py

test/fixtures.py