Change OAV detector snapshots so that hyperion-blueapi does not need to write to /data

Currently the `hyperion-blueapi` process needs to run as `i03-hyperion` because it needs to be able to write to the data directory. This is because the `Snapshot` plugin writes to this directory during plan execution.

We can reduce the privileges of `hyperion-blueapi` if instead we write temporarily to local storage and then in the callbacks read this location and then the callbacks which also run as i03-hyperion can do the write to the data directory.

## Acceptance Criteria
- OAV snapshot plan doesn't need to write to /data
- `hyperion-blueapi` runs as normal user not i03-hyperion