Namelist specification of stencil depths

LFRic's stencil depth specification is currently a bit odd. When online-partitioning, the stencil depth (or depths following #237) are passed to the driver component from an app through the argument list. These depths are generally determined from science options, and in practice this is the `dep_pt_stencil_extent` in the `transport` namelist. On the other hand, the `partitions` namelist in `mesh-tools` has its own `stencil_depth` namelist option.

To resolve this, I suggest we take the following actions:
1. The `partitioning` namelist in the driver component should include a `stencil_depth` option
2. Both the new `stencil_depth` option in the driver component and the existing `stencil_depth` option in `mesh-tools` should be arrays, allowing different depths to be specified for different meshes.

Locate the driver component's partitioning namelist and mesh-tools' partitions namelist, then trace how stencil depths currently enter online partitioning. Confirm how each component represents the existing value before updating both options to accept per-mesh arrays and checking that the driver can read its new option.

Written by the indexing model from the issue text.