feat: add init values for compute and mem allocation in dfx.json

[p-shahi]

Introduces an optional object called initialization_values for the canisters object (if set, can be used in the deploy & install command -- cmd line runtime options have higher priority -- if unset, use system defaults)

  "canisters": {
    "{project_name}": {
      "type": "motoko",
      "main": "src/{project_name}/main.mo",
      "initialization_values": {
        "compute_allocation" : "100",
        "memory_allocation": "8GB"
      }
    },
    "{project_name}_assets": {
      "type": "assets",
      "source": [
        "src/{project_name}_assets/assets"
      ],
      "dependencies": [
        "{project_name}"
      ]
    }
  },

• update documentation

[p-shahi]

We remove the default_value here so that if memory-allocation is not explicitly specified, we will use the value specified in dfx.json.
Setting a default here would mean that when this option is unspecified at runtime, the value of args.value_of("memory-allocation") be 8GB and a dfx.json configured value will never be used