Global param propagation for sequential merger

[thedae]

Adding a way to globally propagate params in the sequential merger without having to specify them in the UrlPattern.

The parameter propagation can be enabled specifying the sequential_propagated_params entry in the ExtraConfig.proxy namespace

Example

    {
      "endpoint": "/example",
      "backend": [
        {
          "url_pattern": "/one"
        },
        {
          "encoding": "no-op",
          "url_pattern": "/two"
        },
        {
          "url_pattern": "/three"
        }
      ],
      "extra_config": {
        "proxy": {
          "sequential": true,
          "sequential_propagated_params": [
            "resp0_id",
            "resp0_profile.name",
            "resp1"
          ]
        }
      }
    }

This configuration would:

• Propagate the field id from /one response to both /two and /three
• Propagate the nested field profile.name from /one response to both /two and /three
• Propagate the full /two response to /three. This only works for no-op encoding backends