No list of uServices available

The repo does not contain a list of the already taken/assigned service identifier.

Creating/proposing a new uService thus requires users to manually scan through the whole repository to figure out the first available identifier from the [defined range](https://github.com/eclipse-uprotocol/up-spec/tree/main/up-l3#3-name--number-registry).

Start by scanning the COVESA/uservices repository for existing service identifiers, then compare them with the linked uProtocol name and number registry. Add a repository list showing the already assigned identifiers and make sure it lets users determine the first available identifier within the defined range.

Written by the indexing model from the issue text.