[LLC] Run `LowLevelCallbackPolicy` per call, not per retry (Azure#1279)

The intention is that this would run per call (and that's how we
documented it in `RequestOptions`) so we have to call the more complex
`HttpPipelineBuilder.Build` overload.

 Update generator to handle the Operation OperationGroup renamed to R…

…estApi (Azure#1277)

* merge from featurev3

* regen after initial shell

* regen after merge

* update to include management core shared files

* updates to pipeline forwarding and final touches on the restapi

* full regen after updates

* update after merge

* update download script

* consolidate targets file

 Configuration does not serialize default values (Azure#1272)

Today we unconditionally write out all default values, which makes the config files very bloated as we add more and more options.

This change adds infrastructure to know what is default, and not write them out (or read them).

If we ever change the default value, this could bite us however.

- Fixes Azure#1246
- Does not change management configuration values for now

# Checklist

To ensure a quick review and merge, please ensure:
- [x] The PR has a understandable title and description explaining the _why_ and _what_.
- [x] The PR is opened in draft if not ready for review yet.
   - If opened in draft, please allocate sufficient time (24 hours) after moving out of draft for review
- [x] The branch is recent enough to not have merge conflicts upon creation.

Use correct header (Azure#1276)

Add CodeGenSuppressTypeAttribute to remove types from codegen (Azure#…

…1275)

* Add CodeGenSuppressTypeAttribute to remove types from codegen

* Address PR comments

* Address more CR comments

Fix null ref issue in ResourceContainerWriter (Azure#1263)

* fix writer bug

* extract as extention method

Co-authored-by: Yeming Liu <yeliu@microsoft.com>

Implement methods in ResourceOperations class (Azure#1273)

Implement ResourceOperations methods

Add autorest changes for singleton (Azure#1262)

* Update swagger

* Add extension writer

- Add Tenant level singleton to swagger
- Fix SubscriptionExtensionsWriter singleton container
- Add SubscriptionExtensionsWriter to emit GetSingletonOperations()

* Fix pipeline

* Merge update

* Update singleton name

* Fix test

Update CODEONWERS for MachineLearning sample (Azure#1269)

 [sample] Update CognitiveServices.TextAnalytics to non preview (Azur…

…e#1267)

- Turns out the sample is on a preview that is no longer valid. Trying
  to run it will cause the service to return errors, which has confused
  me multiple times. Bump it to 3.0 stable.