Taxonomy bucket parser assumes term field was 'term_slug'

In these methods:

https://github.com/alleyinteractive/elasticsearch-extensions/blob/df182448bda51efbdc1438cdda3d1e9f6976d1f8/lib/aggregations/class-taxonomy.php#L87-L124

If `elasticsearch_extensions_aggregation_taxonomy_field` is filtered to another field, like `term_id`, then the term lookup in `parse_buckets()` will fail because `$bucket['key']` contains something other than a slug.