[FEATURE REQ] CosmosDB Spring Data Auditor ReactiveAuditorAware support

**Is your feature request related to a problem? Please describe.**
Using Spring WebFlux, we store the authenticated user in the Reactive context. We would like to be able to automatically write this user to the `@LastModifiedBy` field in our Cosmos entities. This *should* be pretty straightforward since Spring Data has an Auditor concept, which this SDK enables with `@EnableCosmosAuditing`. However, that only looks for a bean that implements `AuditorAware`, which doesn't support getting information from the Reactive context but is designed for getting information out of some sort of thread-local store or the like.

**Describe the solution you'd like**
I'm looking for something along the lines of `@EnableReactiveCosmosAuditing`, which looks for a bean implementing the `ReactiveAuditorAware` interface instead. For example, with Spring Data for MongoDB, there is a `@EnableReactiveMongoAuditing` in addition to `@EnableMongoAuditing`.

**Describe alternatives you've considered**
We could work around it by adding code to each time we save that looks up the current user in the reactive context and sets our modified-by field directly. But it would be much more convenient to just use the Spring Data auditing support which is already there.

Thank you!

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