fix(js): remove default created_at from createExample method.

[tendev-liam]

Fixes issue: #1399

Issue:
Examples created from the createExample method of the JS client are created with 3 digits of fractional seconds.

How Issue Presents:
The endpoint to get dataset versions (https://api.smith.langchain.com/api/v1/datasets/{dataset_id}/version) always responds with 6 digits of fractional seconds.
When filtering expirements by dataset ID the langsmith frontend will use one of the resuls from the get dataset verions i.e the timestamp with 6 digits.
When a expirement is being tagged with a dataset version it will use the modified_at key of an example which if created from the JS client will only contain 3 digits of precison.

Fix:
Hand over the creation of the createExample created_at timestamp to langsmith server.