[nnx] add PathContains Filter

[cgarciae]

What does this PR do?

Adds the PathContains Filter which lets users select paths that contain a specific key. Example

class Model(nnx.Module):
  def __init__(self, rngs):
    self.backbone = nnx.Linear(2, 3, rngs=rngs)
    self.head = nnx.Linear(3, 10, rngs=rngs)

model = Model(nnx.Rngs(0))

head_state = nnx.state(model, nnx.PathContains('head'))

assert 'head' in head_state
assert 'backbone' not in head_state