[Fuseki] Reloadable configuration files

### Version

5.0.0 dev

### Feature

Add a server action that reloads the services defined in a configuration file and replace the server data services configuration with zero-downtime, no HTTP request lost or aborted while the server is running.

This allows:

1. Reloading an in-memory graph with data from a file.
2. Change the location of a TDB2 database
3. Change of database configuration (e.g. (re)setting per dataset or per-endpoint query timeouts)

This is a subset of the configuration file capabilities. Some changes still require a server restart.

Limitation: transient data (in-memory, data loaded via HTTP) may be lost.

### Are you interested in contributing a solution yourself?

Yes

The issue names no files or tests; begin by locating Fuseki's server actions and configuration-file handling. Establish how service reloads can replace the data-service configuration without lost or aborted HTTP requests, and document which changes still require a restart and how transient data loss is handled.

Written by the indexing model from the issue text.