[Fix] added support for XES composite list attributes

[ondrisvu]

Problem: The XES importer would throw an exception when encountering list attributes containing nested composite attributes (meta-attributes or other list attributes)

Solution: Added missing parsing cases to the __parse_attribute function in the XES importer

Additions:

• a minimal failing event log demonstrating the previous error behaviour
• the XesCompositeAttTest test case was added to the existing test suites

Impact: this PR lays the groundwork for extending supported event log formats within the PM4Py library

[fit-alessandro-berti]

Thanks. This is a contribution that from what I understood is going to be part of a larger set of PRs.

We would need to finalize a "contribution level agreement" before merging (as the contribution is non-trivial)