[E-Documents Core] - Supports prefixed root elements in PEPPOL (#7144)

This pull request does not have a related issue as it's part of the
delivery for development agreed directly with @altotovi @Groenbech96

**Implementation**

Addresses an issue where E-Documents using prefixed root elements (e.g.,
ubl:Invoice instead of Invoice) could not be parsed correctly.

The parsing logic now dynamically identifies the XML root element and
uses this information in XPath queries, enhancing compatibility with
various UBL namespace declarations. Includes a new test case to validate
the functionality.

Fixes
[AB#621663](https://dynamicssmb2.visualstudio.com/1fcb79e7-ab07-432a-a3c6-6cf5a88ba4a5/_workitems/edit/621663)

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Co-authored-by: Andrius Andrulevicius <andrius.andrulevicius@companial.com>

Author: GMatuleviciute