LPS-46890 Comments API: transfer all Message Boards references from SearchResultUtil

Hi @sergiogonzalez!

This PR is for the Search subsystem. It contains the full set of changes that extract MBMessage references away from SearchResultUtil. Please review and send back to me with your fixes. I'll incorporate them and resend as small PRs, starting with the tests.

cc @rotty3000 (this is the one that leverages the existing Indexer Registry to provide Search Result Contributors)

cc @marcellustavares (this version already incorporates the fixes from your [first review](https://github.com/arboliveira/liferay-portal/pull/7))

_Reminders:_

All caps `URL` is encouraged in method names and variables
https://github.com/liferay/liferay-portal/commit/212ab9ba77c930feafc68af52fc39f1be9fd7b13#diff-90f7e9d80d74a01a4071a27502998e0bL200