Core, API: Support scanning from a tag

[namrathamyske]

As addressed by #5150 (comment), creating a separate PR for tags.

#5150 will make use of this PR.

@amogh-jahagirdar @rdblue let me know what you think!

[hililiwei]

    SnapshotRef ref = tableOps().current().ref(tag);
    Preconditions.checkArgument(ref != null, "Tag does not exist: %s", tag);
    Preconditions.checkArgument(ref.isTag(), "Ref %s is a tag not a branch", tag);

The error message is modified to be consistent with that in UpdateSnapshotReferencesOperations.

iceberg/core/src/main/java/org/apache/iceberg/UpdateSnapshotReferencesOperation.java

Lines 78 to 96 in f76e9cb

	public UpdateSnapshotReferencesOperation removeTag(String name) {
	Preconditions.checkNotNull(name, "Tag name cannot be null");
	SnapshotRef ref = updatedRefs.remove(name);
	Preconditions.checkArgument(ref != null, "Tag does not exist: %s", name);
	Preconditions.checkArgument(ref.isTag(), "Ref %s is a branch not a tag", name);
	return this;
	}
	public UpdateSnapshotReferencesOperation renameBranch(String name, String newName) {
	Preconditions.checkNotNull(name, "Branch to rename cannot be null");
	Preconditions.checkNotNull(newName, "New branch name cannot be null");
	Preconditions.checkArgument(!name.equals(SnapshotRef.MAIN_BRANCH), "Cannot rename main branch");
	SnapshotRef ref = updatedRefs.get(name);
	Preconditions.checkArgument(ref != null, "Branch does not exist: %s", name);
	Preconditions.checkArgument(ref.isBranch(), "Ref %s is a tag not a branch", name);
	SnapshotRef existing = updatedRefs.put(newName, ref);
	Preconditions.checkArgument(existing == null, "Ref %s already exists", newName);
	updatedRefs.remove(name, ref);
	return this;

[rdblue]

I think we've decided to go with useRef rather than separate tag and branch methods. I'll close this. Thanks, @namrathamyske!