Add custom dataset configuration

As an individual or organization standing up an instance of attack-website with a custom dataset, I want to be able to use exported ATT&CK Navigator layers with an attached ATT&CK Navigator instance hosting the same custom dataset.

Currently, exported layer filles will include the versions.attack field denoting what version of ATT&CK they refer to. However, if a user is hosting a custom dataset they may not have versioned that dataset, creating issues where the Navigator layer cannot match a version in the Navigator's config.versions structure (see https://github.com/mitre-attack/attack-navigator/issues/343).

Add a config setting specifying that a custom dataset has been used, and remove the versions.attack field from output ATT&CK Navigator layers if this config setting is enabled. That will force the output layers to fallback to the most recent version specified in the Navigator instance, e.g the custom dataset.

Start by locating the configuration handling and the code that produces exported ATT&CK Navigator layers. Review how the Navigator's config.versions structure is represented, then trace where versions.attack is added to output. Done means a custom-dataset setting is honored and exported layers omit versions.attack when it is enabled, while existing behavior remains unchanged otherwise.