Add optional monorepo information into schema and post_live_metrics

[mattseddon]

Part of https://github.com/iterative/studio/issues/8848

Required for https://github.com/iterative/studio/pull/9019 / https://github.com/iterative/dvc/pull/10291 / https://github.com/iterative/dvclive/pull/779

This PR adds additional data into the schema to support live experiments from monorepos being displayed correctly in Studio.

We are adding the following to the start event:

    dvc_experiment_parent_data: Optional[Dict[str, Any]] = None,
    subdir: Optional[str] = None,

The rest of the start event remains unchanged. The names of the new fields match the fields already present in Studio for pushed experiments/commits. They are optional so that we can continue to accept "legacy" events from older versions of the client in Studio.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8235015) 100.00% compared to head (4f772d1) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #144 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 312 326 +14 Branches 18 18 ========================================= + Hits 312 326 +14 ```

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