Add API for retrieving the configuration

To aid in troubleshooting, it is almost always useful to be able to read the pipeline configuration of the troubled Logstash. However, accurately gathering this is often problematic because, when asked, users often only provide a subset of their whole configuration. To remedy this problem, it would be useful to be able to have a script gather the configuration from Logstash directly.

However, given that the configuration often has sensitive or secret data (credentials, etc), it would be bad if we exposed the _content_ of the configuration in an insecure way.

Adding security to the Logstash API is a good idea, but out of scope for this ticket.

Until we have a secure API, we can maybe satisfy this by the following ...
## Proposal

Add a new REST endpoint that lists the file paths that were loaded for the current configuration. A script run on the Logstash server could query this API and pack up the these listed files for delivery for support purposes.

Start by locating Logstash's existing REST API entry points and the configuration-loading code, then check how loaded file paths are retained. Define the endpoint response and add coverage for listing paths without exposing configuration contents; the issue provides no file or test names, so repository research is required.

Written by the indexing model from the issue text.