Improve interoperability with Spring Data Rest and other projects.

[michael-simons]

This improves the interoperability with Spring Data Rest by a magnitude by treating @TargetNode as association to the outside world. Why? Because no resource processing needs to be done on @RelationshipProperties to have embedded links for their target node.

Two support classes for both associations and properties have been added as internal API to chim our philosophy from that change.

To demonstrate that change have a look here: https://github.com/michael-simons/neo4j-examples-and-tips/blob/832e0334a09692bdd72474c9ba11390bc68a493b/examples/sdn6-rest/src/main/java/org/neo4j/tips/sdn/sdn6_rest/SpringDataRestConfig.java#L29-L40

This piece is necessary to include the actual Person behind the Actor as an embedded resource. Otherwise, SDR won't recognise it as association, even though it is managed by a repository.

With this change in place, we also can use proper excerpts as recommended by @odrotbohm, for example see here_

michael-simons/neo4j-examples-and-tips@9a74697

A full blown result that shows

• Uniform excerpts on all people
• Correctly embedded people on the actors

{
  "_embedded" : {
    "movies" : [ {
      "title" : "The Matrix",
      "description" : "Welcome to the Real World",
      "actors" : [ {
        "roles" : [ "Emil" ],
        "_embedded" : {
          "person" : {
            "name" : "Emil Eifrem",
            "_links" : {
              "self" : {
                "href" : "http://localhost:8080/people/e43fd0ea-1d26-48ee-b183-a4d74164411e{?projection}",
                "templated" : true
              }
            }
          }
        },
        "_links" : {
          "person" : {
            "href" : "http://localhost:8080/people/e43fd0ea-1d26-48ee-b183-a4d74164411e{?projection}",
            "templated" : true
          }
        }
      } ],
      "released" : 1999,
      "_embedded" : {
        "directors" : [ {
          "name" : "Lana Wachowski",
          "_links" : {
            "self" : {
              "href" : "http://localhost:8080/people/2141c773-86b2-4bab-930a-101de3ba55fc{?projection}",
              "templated" : true
            }
          }
        }, {
          "name" : "Lilly Wachowski",
          "_links" : {
            "self" : {
              "href" : "http://localhost:8080/people/9daf0125-b276-4a1a-9008-6a8e6c1b22ad{?projection}",
              "templated" : true
            }
          }
        } ]
      },
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/movies/The%20Matrix"
        },
        "movie" : {
          "href" : "http://localhost:8080/movies/The%20Matrix"
        },
        "directors" : {
          "href" : "http://localhost:8080/movies/The%20Matrix/directors{?projection}",
          "templated" : true
        }
      }
    } ]
  },
  "_links" : {
    "first" : {
      "href" : "http://localhost:8080/movies?page=0&size=1"
    },
    "self" : {
      "href" : "http://localhost:8080/movies?size=1"
    },
    "next" : {
      "href" : "http://localhost:8080/movies?page=1&size=1"
    },
    "last" : {
      "href" : "http://localhost:8080/movies?page=37&size=1"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile/movies"
    }
  },
  "page" : {
    "size" : 1,
    "totalElements" : 38,
    "totalPages" : 38,
    "number" : 0
  }
}

[meistermeier]

All tests are green ✔️
I like the loose coupling between Spring Data's general associations and our relationship (properties) ✔️
We can now play better with Spring Data Rest ✔️
Haven't found any double space ✔️