Adding support for masking HiPS Layers

[ivan-katkov]

This issue proposes adding the ability to mask HiPS layers based on user-defined criteria.

• For example, given two HiPS maps for flux and SNR, users could dynamically mask regions with low SNR by setting a threshold. This would allow more flexible control over layer visibility.

• A more advanced use case could involve a mask for a target layer (or layers) that depends on a set of other layers and user-defined criteria. It could be a function of input healpix values from these layers, returning a boolean to mask or show healpix from the target layer.