GH-2753: Improve customising Fuseki args

[rvesse]

Improves how Fuseki arguments are customised, in particular makes the ServerConfig class public adding the ability to store extra configuration on it.

It also changes the previously private static applyServerArgs() method into a protected instance method so users can extend FusekiMain and override it to use that to customise the FusekiServer.Builder further.

Unit tests are added that validate that custom arguments can now be passed through CLI inputs and used to influence the Server being built.

GitHub issue resolved #2753

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• [x] Tests are included.
• [ ] Documentation change and updates are provided for the Apache Jena website
• [x] Commits have been squashed to remove intermediate development commit messages.
• [x] Key commit messages start with the issue number (GH-xxxx)

By submitting this pull request, I acknowledge that I am making a contribution to the Apache Software Foundation under the terms and conditions of the Contributor's Agreement.

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See the Apache Jena "Contributing" guide.