feat: keep img tag's class attr (#4050)
This change affects partition html.
Previously when there is a table in the html, we clean any tags inside
the table of their `class` and `id` attributes. However, sometimes there
are images, `img` tags, present in a table and its `class` attribute
identifies some important information about the image. This change
preserves the `class` attribute for `img` tags inside a table. This
change is reflected in a table element's `metadata.text_as_html`
attribute.
