`mapping_sets` endpoint

[anitacaron]

• Mappings listed as a link to the endpoint /mappings_set/{id}/mappings

Fixes #8

[matentzn]

Can you ask for a review by Patrick when you are done?

[anitacaron]

He can start reviewing because I'm unsure of the relative path. @pkalita-lbl

[anitacaron]

Thank you, @pkalita-lbl! I removed the complex function, and I'm using just Request.url_for.

It took me some time because I needed to update the code to be aligned with the new sssom-py release.