Allow use of variants when using add_trait effect

[furran]

Summary

Features "Allow for a "variant" field when using the add_trait effect"

Purpose of change

Makes it so you can specify a variant if you are using u_add_trait or npc_add_trait effect.

Closes #70027

Describe the solution

• "variant" is now an optional field for u_add_trait and npc_add_trait
• if variant is invalid/doesn't exist, it is randomized. No errors are thrown
• specifying a variant for a trait that doesn't have them just adds the trait normally

Describe alternatives you've considered

None

Testing

Used these jsons for testing: variants_test_eocs.json

1. Added the jsons to data\json\effects_on_condition\mutation_eocs
2. used debug menu to run all the eocs in variants_test_eocs.json

Expected/obtained results for each eoc:

• test_add_purple_mohawk: character gains a purple mohawk
• test_add_feline_ears: character gains feline ears
• test_invalid_variant: character gains short hair, but the color is randomized

Additional context