## Description
Begin persisting Exchange delta tokens for data categories that support delta queries. Tokens are stored in a single file as a `map[M365 container ID]token` where both the container ID and token are of type `string`. The file is located in the kopia snapshot that has all the other backup data at the path `tenant-id/{service}Metadata/user/category/delta`. No information about the delta token file is stored in backup details.
## Type of change
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor
## Issue(s)
* closes#1685
## Test Plan
<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E