Graph theory def: Minimal/minimum vertex covers

[SnirBroshi]

Vertex covers for simple graphs exist as SimpleGraph.IsVertexCover.

I suggest we're missing the following definitions:

open Cardinal

def IsMinimalCover (G : SimpleGraph V) (c : Set V) :=
  Minimal G.IsVertexCover c

def IsMinimumCover (G : SimpleGraph V) (c : Set V) :=
  MinimalFor G.IsVertexCover (#·) c

and basic API for them.

[SnirBroshi]

#33032

[SproutSeeds]

Opened focused PR: #36042