* containers: add method to create new {Dict,Set,Array} with custom max_depth
* add vds_tree computation, update tree circuit interface
* add VDTree struct, add DEFAULT_VD_TREE, integrate it with MainPod,EmptyPod,frontend,etc.
* adapt frontend/serialization tests to new containers field (max_depth)
* adapt interfaces to allow using custom vd_tree in frontend & backend constructors
* rename VDTree to VDSet (and derivate namings too)
* containers 'new' always with param 'max_depth', use params.max_depth_mt_containers instead of the global constant MAX_DEPTH
* adapt after rebasing the branch to main latest changes
* apply review suggestions from @ed255
* use emptypod vd_mt_proofs (using vd_set as circuit input), merge the two existing set_targets methods of MainPodVerifyTarget
* document VDSet & vds_root
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