Add Unpaywall fetcher and add JabKit commands for citations

[koppor]

This adds our citation feature to JabKit

Moreover, adds Unpaywall fulltext fetcher.

Steps to test

./gradlew :jabkit:run --args="get-cited-works 10.31083/j.rcm.2018.04.3187"
./gradlew :jabkit:run --args="get-citing-works 10.31083/j.rcm.2018.04.3187"

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[Siedlerchr]

/home/runner/work/jabref/jabref/jablib/src/main/java/org/jabref/logic/importer/fetcher/CustomizableKeyFetcher.java:5: error: cannot find symbol
import jakarta.annotation.Nullable;
-> must be jspecify

[Siedlerchr]

Add getTestUrl for api key check, see e.g. MedlineFetcher

[Siedlerchr]

Have you checked if this work with this check in the ui?

jabref/jabgui/src/main/java/org/jabref/gui/preferences/websearch/WebSearchTabViewModel.java

Line 310 in f83fd80

URLDownload urlDownload = new URLDownload(testUrlWithoutApiKey + apiKey);

[koppor]