Add k4clue to our reconstruction configuration

[TC01]

As a first step towards testing external clustering.

Since this is already in key4hep it should be straightforward to add to our setup.

[TC01]

If you follow the instructions I wrote here you can get k4clue installed on top of 3.1 via spack by:

spack env create k4clue
spack env activate k4clue
spack add k4clue
spack concretize -f --reuse
spack install

On 3.1 this wants to compile ~44 additional packages so it's taking a bit of time to build.

[TC01]

(I was doing this incorrectly in spack, with it added to the correct environment it only pulls in cluestering and alpaka).

This has now been implemented (see #25); we now just need to hook the clusters up to Pandora.