FileSystemProviders from jar-files are not found when running as executable jar generated via spring-boot-maven-plugin

[grmblfrz]

In a project which uses com.github.robtimus:ftp-fs I found that the FileSystemProvider for ftp (and ftps) is not registered when running as executable jar file, so FileSystems.newFileSystem("ftp://...") fails. The identical code works when run in the IDE (intellij).
I've prepared a small test project which demonstrates the problem: fsibug.
I'm aware that it is possible to use the provider directly via new FTPFileSystemProvider().newFileSystem("ftp://...") in my use case so I have a workaround but it has cost me some time to find the problem. Would be great if this could be fixed.

[wilkinsona]

Duplicates #6626.