Fix: make ord document url endpoint configurable and extendable

[zongqichen]

At the moment, there is only one fixed url endpoint set in https://github.com/cap-js/ord/blob/main/lib/defaults.js#L81:

    baseTemplate: {
        openResourceDiscoveryV1: {
            documents: [
                {
                    url: "/open-resource-discovery/v1/documents/1",
                    accessStrategies: [
                        {
                            type: "open",
                        },
                    ],
                },
            ],
        },
    },

and call it in https://github.com/cap-js/ord/blob/main/lib/plugin.js#L20:

  app.get("/open-resource-discovery/v1/documents/1", async (req, res) => {
    try {
      const csn = await cds.load(cds.env.folders.srv);
      const data = ord(csn);
      return res.status(200).send(data);
    } catch (error) {
      console.log(error);
      console.log('Error while creating ORD document');
      return res.status(500).send(error.message);
    }
  });
});

We need to consider two more cases:

1. if user has more than ord documents: e.g they have their own ord document
2. we should make url endpoint configurable when user wants to adjust

[swennemers]

the ORD endpoint configuration is a quite optional feature. the endpoints used in the plugin should be the recommended ones from the specification. Having multiple services, does not mean, we need own ORD documents for each of them. A single ORD document is fine to contain multiple APIs (derived from the multiple services).