Add native save and read functionality to `SSD` and `SPoC` objects

[tsbinns]

Describe the new feature or enhancement

Requested in https://mne.discourse.group/t/no-feature-to-save-an-mne-decoding-ssd-spoc-object/11367

In summary, there is no native option to save and read SSD and SPoC objects. This would be useful to avoid having to re-compute filters over different sessions, like what can be done for ICA filters.

A current workaround is to just pickle the objects, but having this functionality directly in MNE would be useful.

Describe your proposed implementation

Follow the ICA structure with a save() method for saving SSD/SPoC objects and a decoding.read_ssd/spoc() function for loading the saved objects.

Describe possible alternatives

N/A

Additional context

No response

[larsoner]

Sounds reasonable to me!