Add `Flow.metadata` attribute used in `Flow.update_metadata` method

[janosh]

Flow.update_metadata is now much more similar to and has reached feature parity with Job.update_metadata:

https://github.com/materialsproject/jobflow/blob/367836b0d51a89fa58a9d685c4cdb9fc7f806c01/src/jobflow/core/job.py#L922-L930