…t#6393)

### Motivation and Context

When using SemanticTextMemory.SaveReferenceAsync, no value is passed for
the `Key` to use. In the Cosmos DB Memory Connector, we treat the "Key"
as the Id and PartitionKey, and so it has to have a value. Address this
by generating a `GUID` to use if none are provided.

### Description

* Assign a GUID key when the passed in `MemoryRecord.Key` is null or
empty so that CosmosDB always has a valid PartitionKey.
* While adding test coverage for this I noticed two things:
1. `withEmbeddings` is ignored in
`AzureCosmosDBNoSQLMemoryStore.GetAsync`. It's not clear whether it's
worth changing this, as point reads like the current one tend to be less
expensive than queries in Cosmos DB.
2. The SimilarityScore returned from `GetNearestAsync` wasn't actually a
similarity, it was a cosine distance. Normalize that.

Fixes microsoft#6379.

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>