feat(jabsrc): add endpoint for adding an entry to the currently opened library

[InAnYan]

Closes https://github.com/JabRef/jabref-issue-melting-pot/issues/676

Steps to test

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[trag-bot]

The try block covers too many statements, which can make it harder to identify which specific statement caused an exception.

[koppor]

Use three slashes 😅

[koppor]

I think, this won't work...

REST works differently... a) redirect to path where this is stored -- I don't know how the client works or b) direct handling in LibraryResource.

Conflicts with @Path("libraries/{id}") at libraries

REST background: ROA -> https://en.wikipedia.org/wiki/Resource-oriented_architecture - everything is a resource, not a command endpoint (if possible)

i will dive into later

[Siedlerchr]

A good read on REST https://github.com/stickfigure/blog/wiki/How-to-%28and-how-not-to%29-design-REST-APIs

[InAnYan]

Well, ideal:

1. Fetch /libraries?sort_by=date&sort_direction=desc
2. /libraries/#{libraries[0].id}

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨

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