[ftr] XQuery customisation

[bwbohl]

Is your feature request related to a problem? Please describe.
In various projects using Edirom Online, there has always been a need to customise the exact query location within MEI or TEI files to generate the desired output for the Edirom Online GUI.

Describe the solution you'd like
A potential solution could be a defined location within an edition package or the database (cf. #241) where such customisations could live. This could significantly enhance the flexibility and usability of Edirom Online, particularly in the context of XQueries, where an edition-custom.xqm XQuery module, in combination with a lookup function, could be a powerful tool.

Describe alternatives you've considered
An alternative could be to define an API for edition-data-endpoints, i.e. something [ENDPOINT]/music-sources/[ID]/window-title to retrieve the desired window title. In this way, the data-query logic would be entirely on the data-provider side and nothing Edirom Online has to worry about.

Additional context
While both approaches have their merits, I favour the second one. However, I acknowledge that it might put a significant workload on the potential users of Edirom Online. This brings me to an additional project: basic implementation of the API endpoints in XQuery provided as an eXist-db library package. This the could be used as default for providing the correct results while the installation of Edirom Online an edition data in the same database would stay the way it is right now.

[bwbohl]

I deem the API solution rather feasible in the context of a 2.0.0 release, thus the custom-xquery-module and lookup could be the interim solution.