Add an iris-esmf-regrid based regridding scheme

Description

Add an iris-esmf-regrid-based regridding scheme and use it as the default scheme for unstructured grids when linear or area_weighted is specified in the recipe. For nearest, we need wait for https://github.com/SciTools-incubator/iris-esmf-regrid/pull/276 and https://github.com/SciTools-incubator/iris-esmf-regrid/pull/369 and a new release of iris-esmf-regrid before this is useful.

Example usage (as generic scheme) in preprocessor in recipe:

regrid:
  target_grid: 1x1
  scheme:
    reference: esmvalcore.preprocessor.regrid_schemes:IrisESMFRegrid
    method: bilinear
    mdtol: 0.7

Example plots for regridding 2D variable tos to a 1x1 degree grid are available here. Example plots for regridding 3D variable thetao to a 1x1 degree grid and subsequently extracting the 500m depth level are available here.

Part of #2405.

Links to documentation:

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