Add HTTP server for MS Office Word integration

[tobiasdiez]

Adds HTTP server for Office integration. At the moment, it's only a prototype showing that we do have enough space in our module list to include the jdk.httpserver (which is unfortunatly not as dev-friendly as other solutions, but works fine).

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

See my otherPR koppor#628 for setting up a nice REST API

[tobiasdiez]

@Siedlerchr the point here is that jdk.httpserver is small enough to be included while jetty/tomcat and (probably undertow) are too large. Thus, in the short term this seems to be our only option.

[koppor]

Sacrificing code maintainability because of a bug in the language environment 🙈

[tobiasdiez]

It's not that terrible, just a bit more low-level and can be abstracted away. But yes, if that's what it takes to play around the annoying jdk bug but have a nice user experience, then I'm fine with it.

[Siedlerchr]

We need to have SSL support enabled for the office addin

[koppor]

Superseeded by #9748