Add --fixture CLI flag for catalog fixture mode

[ulises-jeremias]

Feature Request

Add a --fixture CLI flag to create-awesome-python-app so users can switch to fixture/catalog-test mode via CLI, not only via the CPA_CATALOG_FIXTURE env var.

Current behavior

Only CPA_CATALOG_FIXTURE env var and CPA_FIXTURE_DIR work. There is no --fixture [dir] CLI flag.

Expected behavior

cpa --fixture # use catalog fixture from CPA_FIXTURE_DIR or default
cpa --fixture ./my-fixtures # use custom fixture directory

Reference

CNA has --fixture [dir] flag (+ CNA_FIXTURE_DIR env var). Parity gap documented in the feature alignment sweep.