Add Async Methods to BaseDmsProvider and Dependent Classes

[jochenwezel]

To address the performance bottlenecks described in #8, implement asynchronous methods in BaseDmsProvider and all dependent classes. This enhancement aims to facilitate improved parallelism and responsiveness, especially when handling directories with many sub-directories.

Tasks:

• Identify all synchronous operations in BaseDmsProvider and dependent classes.
• Refactor these operations to provide Async alternatives (e.g., Task-based methods).
• Update consumers of these classes to utilize the new async methods where beneficial.
• Ensure backward compatibility if possible.
• To reduce duplicate code and for better code maintenance, old sync methods should call async methods where beneficial (also in depending classes)
• Add or update unit tests to cover async workflows.

This is a sub-issue of #8 and addresses the proposal for a more asynchronous design.