<!-- PR description-->
* Attachment download needed an expand operation. Added that.
* Added test coverage with gock. Tested with manual backup of posts which contain attachments(embedded/non-embedded). We can't add e2e tests with attachments, since API to create new conversations requires delegated access.
* Note that https://github.com/alcionai/corso/issues/4991 is still an open item. We don't have a resolution for it right now, since attachment endpoint requires delegated token. Defaulting to let the backup fail for "too many attachments" error case. We don't know yet if we'd see that with group mailboxes, and whether it'd be the same error that we saw with exchange (recurring 503s).
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#### Does this PR need a docs update or release note?
- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>
#### Test Plan
<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
1. moves the m365/graph package from internal to pkg/services/api so that options are accessible to sdk users.
2. exposes graph.Options in the api client.Service call.
---
#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🌻 Feature
adds basic get requests for the conversations api set to the api package. Plust some other minor change for formatting, correctness, or necessity.
---
#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🌻 Feature
#### Issue(s)
* #4536
#### Test Plan
- [x] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E
