Bulk actions take tag_id, not tag

In what may be the most niche breaking change ever, we've changed the tag field in the metadata field of bulk actions to tag_id.

Previously, when you wanted to create a bulk action to add a tag to a list of subscribers, you could either specify a list of tag IDs:

{
    "type": "apply_tags",
    "metadata": {
        "tag_ids": ["123", "456", "789"],
        "action": "add"
    }
}

Or you could specify a single tag:

{
    "type": "apply_tags",
    "metadata": {
        "tag": "123",
        "action": "add"
    }
}

We've unified the two approaches to use tag_id instead of tag:

{
    "type": "apply_tags",
    "metadata": {
        "tag_id": "123",
        "action": "add"
    }
}

The behavior of the endpoint has not changed, and we cut a new version of the API to make this backwards-compatible for existing callers.