[feature] change the invalid scheme `xmldb:exist:` to `xmldb+exist:`

[line-o]

Is your feature request related to a problem? Please describe.

With the status quo all URIs that start with xmldb:exist: are invalid as the colon character : is not allowed within a scheme
(see https://datatracker.ietf.org/doc/html/rfc3986#section-3.1).

Since + character is used widely in other contexts to signal a combination of protocols it might be a good way forward.

I am used to use git+https:, for example.

Describe the solution you'd like

eXist-db to only use valid URIs.

Describe alternatives you've considered

Alternatively, we could drop the exist: portion completely making all URIs just start with xmldb:, if it does not add anything to a URI. So far I have not seen a URI that needs to be xmldb:exist: - xmldb: works as well.
Maybe someone with knowledge of the history behind the decision to recognise xmldb:exist: could chime in here.

Additional context

This is a breaking change whatever direction we will take. So it cannot be back ported to any released major version.
As a benefit, all URIs will be testable and verifiable without having to rely on custom validators and this will allow to further simplify our codebase and reduce a class of errors.

[reinhapa]

@line-o this URI is defined by the XMLDB API here

[adamretter]

Nice idea, but we tried this in the past and gave up - it broke all the things