Extend file_list to Include Category Paths

[lasuk]

Extend file_list to return a pandas data frame, including a recursive path of the category the file is associated with.

• The file path shall be returned as a string imitating a file system, with slashes '/' separating parent from child categories.
• A leading slash '/' indicates the root directory.
• If available, file size, last modified date and mime type shall be returned as additional data frame columns.

CashCtrlApi.file_list(self) -> pd.DataFrame

Please test this method with a test in the test suite.