Set missing data to zero value in make_consistent_with_lsm

# ✨ Feature Request
In the regridder there is an option to make the result consistent with a mask, i.e. have missing data on specified points. The request here is that there is a option to set these missing data points to zero.

## Requirements
Optionally set missing data to zero in the function make_consistent_with_lsm, and plumb this through to the CLI of the general regrid script.

## Context
The main use of this feature would be where a field has a land sea split in values, but is required to have zero values in the "false" side. This can happen when the science in the model does not have an "if lsm" clause around the use of the data and zero gives a null impact. An example of this is the deltasst ancillary.

Start by locating the regridder function make_consistent_with_lsm and the CLI entry point for the general regrid script. Trace how the existing mask-consistency option is exposed, then add an optional zero-value setting through both interfaces and verify that masked points become zero when enabled while existing behavior is unchanged by default.

Written by the indexing model from the issue text.