P2322R6 accumulator types for scan and reduce by key

[gevtushenko]

This PR addresses the following issue. It's based on the previous PR so the first commit might be skipped during this review.

⚠️ Breaking changes:

1. Device scan, scan by key, and reduce by key now do not work with device extended lambdas by default.
2. AgentReduceByKey now accepts accumulator type as the last template parameter.
3. AgentScan now accepts accumulator type as last template parameter.
4. AgentScan::ConsumeRange now accepts prefix as accumulator type instead of output iterator value type.
5. AgentScanByKey now accepts accumulator type as the last template parameter.
6. DeviceReduceByKeyKernel now accepts accumulator type as the last template parameter.
7. DispatchReduceByKey now accepts accumulator type as the last template parameter.
8. DeviceScanKernel now accepts accumulator type as the last template parameter.
9. DeviceScanPolicy now accepts accumulator type instead of output iterator value type.
10. DispatchScan now accepts accumulator type as a template parameter.
11. DeviceScanByKeyKernel now accepts accumulator type as a template parameter.
12. DeviceScanByKeyPolicy doesn't accept initial value type.
13. DeviceScanByKeyPolicy accepts accumulator type as a template parameter.
14. DispatchScanByKey now accepts accumulator type.

[alliepiper]

Some comments inline. This will also need a breaking changes summary (APIs, behaviors) in the description.