Core: populate target branch in snapshot property

[yingjianwu98]

Motivation
Currently, when a snapshot is created in Iceberg, there is no metadata indicating which branch it was committed to. This information is lost
After the commit completes, making it difficult to:

• Audit snapshot provenance: Users cannot determine which branch created a particular snapshot
• Debug branch operations: When investigating issues with multi-branch workflows, the origin branch is unknown
• Implement branch-aware tooling: Monitoring and observability tools cannot track which branches are most active or analyze branch-specific metrics

Previously, I have a PR to only include it in wap branch but I feel this is useful for general branch use case and the current PR is probably much cleaner.