[OCI] lazy import

[asaiacai]

Ran into this while testing digital ocean. These imports shouldn't be breaking serve/jobs when I've only enabled clouds other than oci. this fixes that.

Traceback (most recent call last):
  File "/root/skypilot-runtime/lib/python3.10/site-packages/sky/clouds/oci.py", line 466, in get_credential_file_mounts
    oci_cfg = oci_adaptor.get_oci_config(
  File "/root/skypilot-runtime/lib/python3.10/site-packages/sky/adaptors/oci.py", line 26, in get_oci_config
    oci_config = oci.config.from_file(file_location=conf_file_path,
  File "/root/skypilot-runtime/lib/python3.10/site-packages/sky/adaptors/common.py", line 46, in __getattr__
    return getattr(self.load_module(), name)
  File "/root/skypilot-runtime/lib/python3.10/site-packages/sky/adaptors/common.py", line 36, in load_module
    raise ImportError(self._import_error_message) from e
ImportError: Failed to import dependencies for OCI. Try running: pip install "skypilot[oci]"

Tested (run the relevant ones):

• Code formatting: bash format.sh

[romilbhardwaj]

This should be fixed by #4178 :)

[asaiacai]

thanks! closing