[GQL] extensions.current_fuel_block_height uses js number instead of scalar

[Voxelot]

Elsewhere in the GQL api we use a scalar value to represent the block height, however in the gql extension that provides the block number of each request it uses a plain js number. We should aim to use the same types for representing block height where possible within the GQL api.

example response:

{
  "data": {
    "blocks": {
      "edges": [
        {
          "node": {
            "header": {
              "height": "42531359"
            }
          }
        }
      ]
    }
  },
  "extensions": {
    "current_consensus_parameters_version": 7,
    "current_fuel_block_height": 42531359,
    "current_stf_version": 32
  }
}

[MitchTurner]

@Voxelot I'm not sure I understand this task.

Extensions is a map between String and Value where

#[derive(Clone, Debug, Eq)]
pub enum ConstValue {
    /// `null`.
    Null,
    /// A number.
    Number(Number),
    /// A string.
    String(String),
    /// A boolean.
    Boolean(bool),
    /// A binary.
    Binary(Bytes),
    /// An enum. These are typically in `SCREAMING_SNAKE_CASE`.
    Enum(Name),
    /// A list of values.
    List(Vec<ConstValue>),
    /// An object. This is a map of keys to values.
    Object(IndexMap<Name, ConstValue>),
}

Do we want to represent the block height in extensions as a String, so it shows up the same? I don't think we can set it as a scalar.