Add metadata field to Slot class/configuration dict

adding a dict to the slot as metadata would be extremely useful to record information that are not strictly related to how to build the slot, but could be used to enhance what we display on web pages.

For example we could have:

• a flag to state that the slot should not appear in the main summaries page (e.g. ref and mr slots are not very useful in that page)
• record the name and id of reference slot build for a MR (/ci-test) job

In principle, also the description of a slot would go in the metadata section, but it can be done at a later stage.