Support jumping to jdt:// URIs with GoTo

[christolis]

This pull request addresses a limitation in ycmd's integration with jdt.ls, where jumping to definitions in external JAR files (e.g., project dependencies) resulted in errors like "Cannot jump to location." (look at the before vs after videos below). By enabling the classFileContentsSupport extended capability during initialization, ycmd signals to jdt.ls that it can handle jdt:// URIs.

More information on the commit messages.

Note! This PR should be merged alongside #4302 on the YouCompleteMe repository.

Before vs After

https://github.com/user-attachments/assets/5d6d4e92-d52d-45a7-a964-ca0b1d367c42

https://github.com/user-attachments/assets/960465b6-f447-4882-bafd-9c99d0e65708

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This change is