Hotfix for MLFlow validator spans during async execution

[CalebCourier]

This PR addresses #1163

To avoid orphaned spans being spawned from the Validator.validate calls executed in the ThreadPoolExecutor, we check if there is a current span and do not wrap the validate call in a new span if there isn't.
In conjunction with this, we now also wrap Validator.async_validate this does not experience the same issue as listed above because it itself is not run in the executor, rather it is the one running the executor.

This is not a long term solution for the root cause, but it does avoid the behaviour we see in the issue.

Other potential issues from the ThreadPoolExecutor are any validators that attempt to retrieve information from the ContextStore as it may not have the context that existed in the main thread. This would need to be tested and verified.

[notion-workspace]

Validator spans are orphaned from parent spans when running in async