Reduce FileExtension ambiguity

[shreyamalviya]

Description

As of now, in the guardicore/monkey project, FileExtension is only relevant in the ransomware plugin. However, the plugin defines a FileExtension of its own which uses pydantic (the definition in this repository doesn't) and has more constraints.

Do one of the following:

1. Update the FileExtension in this repository with the definition in the plugin (be sure to update the plugin as well)
2. Remove it from here and move FileExtension from the plugin to this repository when it needs to be used in some other place