[Enh]: Add REST keyword `$count`

## What?

`/Products?$filter=Price gt 50&$count=true`

## Why?

Provide simple aggregate for queries.

## How?

```json
{
"@odata.count": 124,
"value": [ ...items... ]
}
```

Start by locating the REST endpoint handling for `/Products?$filter=Price gt 50&$count=true` and the existing `$filter` query processing. Trace how query results are returned, then verify that a request with `$count=true` produces an `@odata.count` value alongside the `value` array, as shown in the issue.

Written by the indexing model from the issue text.