search: add structural search service

Created by: rvantonder

Summary

• Adds structural searcher implementation. Comby is called when the searcher endpoint gets a request where patterntype:structural is set
• Adds tests for basic functionality

There are a bunch of things that need polishing that is not handled in this PR (like highlighting and handling other fields). These are omitted intentionally and will be handled over time. e2e integration tests to follow.