[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]

some small things! 🦁 🐰 🦒 🐃 🐊 🦊

[dburkhart07]

few small things, but should be good after, so ill approve ig! 🧵 🛍️ 👕 🚗

[sam-schu]

how come we stopped calling validateId?

[amywng]

you're commenting on outdated code, but i think for the equivalent of what you're commenting on it's cuz findByIds in the users service validates them