GH-1502 Create experimental Spring Boot standalone executable for server

[benherber]

GitHub issue resolved: #1502

Briefly describe the changes proposed in this PR:

This PR introduces another build for the RDF4J server outside of the standard WAR packaging. There are a myriad of use-cases specified in the issue but some major benefits include: (i) being more in line with modern deployment (i.e., cloud) strategies; (ii) being more extensible for third parties without having to break WARs open; and (iii) simplifying configuration and Middleware integration using Spring Boots configuration as code approach rather than XML-based.

This is considered an exploratory effort on my end to start the ball rolling and therefore is isolated to a new maven module and is marked with the @Experimental annotation accordingly. The eventual hope would be to pull the spring boot configuration eventually down into the rdf4j-http-server-spring module and have other modules such as rdf4j-http-server and rdf4j-server-jar build WAR and JAR artifacts respectively. However, since this would most likely break any third-party server extensions (servlet versions, war-patching, etc.), this would most likely need to wait for a major release.

Note that this is incomplete (there are the homepages and some of the more frivolous server niceties yet to wire in, but the core api / protocol is implemented), happy to keep in a feature branch until deemed stable / feature-complete; I just wanted to start gathering feedback as early-on as possible.

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