Improve handling of `FilePath` inside `PimegaCam`

Making this an issue to better keep track of the discussion surrounding it.

> @flowln just a little update about the file_path discussion. I talked with @RafaelLyra8 and, at least in the cases of PNR and SPU, we need to set and validate the file path using components from `PimegaCam`, that is why I was not using (and didn't know about) this `HDF5PluginWithFileStore`. So maybe, it would be useful a component that already checks the file_path when we set it trough the `file_path` Component from the `PimegaCam` class.
Anyway, I'm extending myself in a discussion that was not meant to be happening in this PR kkkkkkk again so sorry for this, I'm still learning how to correctly contribute in the projects. Please, if you still have some thoughts about this topic, feel free to reach out! Thank you guys so much for the help.

_Originally posted by @joao-biondo in https://github.com/cnpem/sophys-common/issues/87#issuecomment-4270206235_

Start by reviewing the linked discussion about `file_path`, then inspect the `PimegaCam` component and the role of `HDF5PluginWithFileStore`. Clarify the expected behavior for PNR and SPU, including how the path is set and validated; the work is done when that behavior is agreed and covered by the relevant tests.

Written by the indexing model from the issue text.