chore: Use SnapshotRefType Enum instead of hard-coded strings (#2880)

# Rationale for this change
Use the enum instead of hard-coded strings!

Everywhere else that `"branch"` and `"tag"` are used seems to be in
tests so I think fine to leave those alone

## Are these changes tested?
No functional changes

## Are there any user-facing changes?
No

Author: jayceslesar

pyiceberg/table/update/snapshot.py

@@ -880,7 +880,7 @@ def create_tag(self, snapshot_id: int, tag_name: str, max_ref_age_ms: int | None
         update, requirement = self._transaction._set_ref_snapshot(
             snapshot_id=snapshot_id,
             ref_name=tag_name,
-            type="tag",
+            type=SnapshotRefType.TAG,
             max_ref_age_ms=max_ref_age_ms,
         )
         self._updates += update
@@ -921,7 +921,7 @@ def create_branch(
         update, requirement = self._transaction._set_ref_snapshot(
             snapshot_id=snapshot_id,
             ref_name=branch_name,
-            type="branch",
+            type=SnapshotRefType.BRANCH,
             max_ref_age_ms=max_ref_age_ms,
             max_snapshot_age_ms=max_snapshot_age_ms,
             min_snapshots_to_keep=min_snapshots_to_keep,