Build a cache path for an API spec
See also
bunddev_spec() to download and parse specs.
Other OpenAPI:
bunddev_cache_dir(),
bunddev_call(),
bunddev_call_tidy(),
bunddev_endpoints(),
bunddev_ms_to_posix(),
bunddev_parameter_values(),
bunddev_parameters(),
bunddev_parameters_for(),
bunddev_spec(),
bunddev_timestamp_to_ms()