feat: generate OLX with single-quoted attributes (WIP)

[kdmccormick]

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[kdmccormick]

Since the latest iteration of the Content libraries upstream sync plan doesn't involve stuffing JSON into an OLX attribute, so I'm closing as this is no longer a priority.

However, if this is ever picked back up, here a summary of feedback from @ormsbee and @bradenmacdonald :

• This would be good for OLX readability...
• but it's a shame that there's builtin LXML option to do this for us...
• although it's good that the logic here is at least isolated to one function in one module.
• Having single quotes is not worth risking content integrity, so it would need to be airtight.
• One way to make it failsafe would be to do a round-trip check when serializing, and fallback to the original double-quoted XML if it's not an exact match:

  xml_double_quotes = etree.tostring(root_node, ...)
  xml_single_quotes = "".join(_convert_attrs_to_single_quotes(iter(xml_str)))
  root_node_single_quotes_roundtripped = etree.fromstring(xml_single_quotes)
  if root_node_single_quotes_roundtripped == root_node:
      return xml_single_quotes
  else:
      return xml_double_quotes

• Performance impacts of the above would need to be considered. Roll out with a waffle flag and monitor?