Pull eiger pixel mask into PVs

To make metadata more easily readable from DAQ it would be good if it could be pulled into PVs alongside being written to the meta file. The first bit of data that would be useful is the pixel mask, there will likely be others but once we have the process in place they will hopefully be easier.

## Acceptance Criteria
- There is a PV I can read that I can read that gives the pixel mask

No file, test, or entry point is named. Start by locating where the pixel mask is written to the metadata file and how existing PVs are defined or exposed, then trace the DAQ-facing metadata path. Done means a readable PV exposes the pixel mask alongside the metadata file output.

Written by the indexing model from the issue text.