[services] rename registration table to users table

Summary: we're going to repurpose this table to store more than just registration data (i.e. username, wallet address, auth type, user's public key). this rename is the first step

Test Plan: ran the service locally and made sure i could read/write to the renamed DDB table

Reviewers: atul, geekbrother, palys-swm, karol-bisztyga

Reviewed By: palys-swm, karol-bisztyga

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4559