Add search feature to webmvc-http sample

[jvalkeal]

Thought I'd create this PR if you find it useful as I wanted to see what comes out with union types as no samples uses it.

• This is not to add any kind of sophisticated search feature but to
  have unions in a query result to see what comes out with example like:

    query {
      search(text: "spring", type: ARTIFACTREPOSITORY) {
        ... on ArtifactRepository {
          name
        }
        ... on Project {
          name
        }
      }
    }

where you can use text as search field for names and optionally use type
to limit results either to ARTIFACTREPOSITORY or PROJECT.

[rstoyanchev]

As the Boot starter is about to move (see #207), the samples need to be resettled (see #208). Some will become integration tests within Spring GraphQL (without Boot), we'll be adding getting started guides on spring.io, and perhaps have one more involved sample. So we'll keep this on hold for now.

[jvalkeal]

Yeah, I didn't see those issues until later. Makes sense.

[rstoyanchev]

We'll consider this as part of #208, although I'm not sure there is much, related to unions, that's specific to Spring for GraphQL.