MM-45120: Track team join times. Add API to retrieve new team members since a given time. (#20708)

* MM-45120: Starts tracking team join time. Adds API to retrieve team members who joined after a given time.

* MM-45120: Updates json casing to match model.User.

db/migrations/postgres/000092_add_createat_to_teamembers.down.sql

@@ -0,0 +1 @@
+ALTER TABLE teammembers DROP COLUMN IF EXISTS createat;

db/migrations/postgres/000092_add_createat_to_teamembers.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE teammembers ADD COLUMN IF NOT EXISTS createat bigint DEFAULT 0;
+CREATE INDEX IF NOT EXISTS idx_teammembers_createat on teammembers (createat);