Add ReadDecorHandleKey to ensure that MuonTruthAssociationAlg is scheduled after TrackParticleTruthAlg (ATLASRECTS-5348)

Hi,

previously, the MT scheduler did not know that the MuonTruthAssociationAlg has to be executed after the TrackParticleTruthAlg since it needs the CombinedMuonTrackParticles.truthParticleLink to be present (cf. ATLASRECTS-5348). Now, by adding a ReadDecorHandleKey (for CombinedMuonTrackParticles.truthParticleLink) to the MuonTruthAssociationAlg, this dependency is reported to the scheduler, thus, the MR fixes ATLASRECTS-5348.

Adding @wleight and @emoyse to check

Adding @ssnyder and @akraszna since we were discussing this in the reco meeting

Best, Nico