[MLflow] Support cost attribution for arbitrary span types and visualize them in the UI

<!-- issue-warning --> > [!WARNING] > Before submitting a PR, please make sure that: > - A maintainer has triaged this issue and applied the `ready` label > - This issue has no assignee > - No duplicate PR exists > > PRs not meeting these requirements may be automatically closed. ### Willingness to contribute Yes. I can contribute this feature independently. ### Proposal Summary MLflow recently introduced built-in tracking of token usage and cost for LLM spans. This proposal extends that capability so any span type can declare a standardized cost attribute, and MLflow automatically visualizes these costs similarly to the existing UI for inference span costs. Specifically, the feature would: - Allow any span to define a standardized cost attribute (e.g., `mlflow.span.cost`) - Include these costs in trace-level total cost aggregation - Display span cost in the UI - Include non-LLM span costs in experiment-level cost charts This would allow MLflow to track costs not only for LLM inf