[SSF-171] update volunteers endpoint

[amywng]

ℹ️ Issue

Closes SSF-171

📝 Description

• changed updatePantryVolunteers to patch and to take in add and remove id lists in pantries.controller.ts
• updated service function to add and remove specified users from pantry volunteers, throwing if given id lists overlap at all, if any of the given ids are not volunteers or don't exist, or if the pantryId doesn't exist
• updated controller and service tests

✔️ Verification

Checked with Postman that adding/removing worked and threw expected exceptions when needed. Verified written tests passed

[dburkhart07]

some funny dsa adjustments! 🎱 🦀 📱 👑

[dburkhart07]

While we are on the topic of sets, on conversion, we should validate at the start that there are no duplicates within our respective lists (cant remove a volunteer twice and cant add one twice), and add a test for this.

[dburkhart07]

Can we actually change this to use findBy({ id: In(allVolunteerIds) }) so that we only need to do one query select?

[dburkhart07]

We don't need to check that the volunteer users being removed are volunteers (we already know they are), so we can just check the ones being added.