Allow customizing the location of the e2e project when running the @nx/plugin:plugin generator

[tinesoft]

When running the @nx/plugin:plugin generator with e2eTestRunner option different from none, the generator will generate the accompanying e2e project within the same directory as the plugin itself by default.

While this a valid default location, it would be great if user could change that location (for example to gather all e2e projects in a e2e/ directory).

We could introduce a new e2eProjectDirectory option, which, when set, will enable such behavior.