Add metadata field into BranchContents and TagContents

[majin1102]

Like #6333

I think adding an optional metadata field to both tags and branches would help management platforms use Lance itself as the single source of truth for tag and branch metadata.

That said, this is less urgent than #6333, so I’m opening a separate issue to discuss it independently.

Two additional points to consider:

1. We could make this metadata structured, for example as JSON, so it can hold richer metadata instead of just plain text.
2. If we decide to move forward with this, we may need to evaluate whether introducing an API-breaking change in dataset.rs is acceptable. Adding a separate API solely to support an extra description field feels a bit
   awkward to me.