Custom value mapping for Boolean attributes

[yannick-leu]

Description

For Boolean attributes, implement the option to map specific input values to true/false in the mapping configuration (e.g. that "yes" is mapped to "true"). This kind of custom mapping is already possible for Enum values but not Boolean. The UI should be similar to the custom Enum mapping UI.

In v2.8.0, if a value is imported that doesn't match any of the configured mapping options, it will be treated as if it was mapped to false. Instead, the import should be unsuccessful and result in an error with an appropriate error message. This way, it makes it harder to accidentally import typos and is also consistent with how custom Enum mapping works.

Motivation

Users of Excel Importer might be working with Excel files that contain other Boolean values than true/false, such as yes/no or X/(empty). Instead of having to correct every differing Boolean value in the file, it would be simpler if they could just configure the mapping, like they already do with Enum values.

Proposed Solution (optional)

The custom Boolean mapping could be implemented similar to the current implementation of custom Enum mapping.