Add Stats to DescribeTaskQueue (#596)

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<!-- Describe what has changed in this PR -->
**What changed?**

Added `TaskQueueStats` field to `DescribeTaskQueueResponse`.

<!-- Tell your future self why have you made these changes -->
**Why?**

To provide non-versioning users with relevant task queue stats.

PS: The long-term plan is to deprecate the ENHANCED mod fields.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**

N/A

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right after this PR was merged. -->
**Server PR**

temporalio/temporal#7816

Author: stephanos