Add option to reduce opacity of residues not found in 3D structure mapping

[prihoda]

Is your feature request related to a problem? Please describe.

I would like to inform the user about which parts of the structure exist in the related sequence, and which parts are "extra". This is relevant for us because we are visualizing sequence-structure pairs that are not 100% matches, so only a part of the sequence will be covered by the structure and vice versa.

Describe the solution you'd like

I would like to add an option to protvista-structure like unmapped-residue-opacity="0.5" that reduces opacity of residues not found in the sequence-structure mapping json.

The result could look something like this (mockup):

Describe alternatives you've considered

N/A

Additional context

N/A

[prihoda]

I'm not sure how complex this would be to implement, this is only a feature request at this stage.

@xwatkins can you comment please? I see you implemented the AF2 color scale, I assume this would be similar, except we would be changing opacity based on a list of residue ranges (e.g. [[1,50], [100,150]])