[feature] allow multiple edge builders

[JPXKQX]

This PR extends the current functionality to support multiple edge builders between 2 sets of nodes.

• The edge_builder field is renamed to edge_builders, which now requires a list.
• The source/target_mask_attr_name fields are now specified inside each edge builder.

The current implementation lacked support for custom limited-area graphs, which required 2 cutoff_factor's to connect nodes both within the limited area and along its boundary. They could be instantiated twice in the list of edges. However, edge attribute normalization occurs after each block. Then, we would have to define edge attributes outside of the edge definitions, leading to a loss of flexibility.

One new functionality that had to be implemented is concat_edges() to concatenate the two sets of edges, dropping the repeated edges.

The current config file for a LAM experiment would look like this (including only nodes & encoder edges):

nodes:
  data:
    node_builder:
      _target_: anemoi.graphs.nodes.ZarrDatasetNodes
      dataset: 
        cutout:
        - dataset:${hardware.paths.data}/${hardware.files.dataset}
          thinning: 25
        - dataset: ${hardware.paths.data}/${hardware.files.forcing_dataset}
        adjust: all
    attributes:
      cutout_mask:
        _target_: anemoi.graphs.nodes.attributes.CutOutMask
  hidden:
    node_builder:
      _target_: anemoi.graphs.nodes.LimitedAreaTriNodes
      resolution: 5 
      reference_node_name: data
      mask_attr_name: cutout_mask

edges:
  - source_name: data
    target_name: hidden
    edge_builders:
    - _target_: anemoi.graphs.edges.CutOffEdges
      cutoff_factor: 0.6 # only for cutoff method
    - _target_: anemoi.graphs.edges.CutOffEdges # connects only boundary nodes
      cutoff_factor: 6 # only for cutoff method
      source_mask_attr_name: cutout
    attributes:
      edge_length:
        _target_: anemoi.graphs.edges.attributes.EdgeLength
        norm: unit-std

NOTE: This change will break compatibility with graph configurations of previous versions.

[HCookie]

From a code perspective, this looks good to me, however, I am not familiar with the overall design of graphs, so I would suggest getting another review focusing on that.