ArcGIS: read item metadata XML for field labels and definitions #41

Problem

get_schema now reports each field's name, type, alias, string length and domain (#38). Many ArcGIS items also carry formal metadata (ISO 19139 / FGDC / ArcGIS format). Its entity-and-attribute section defines each field: a label, a definition, units and sometimes value ranges. Aliases rarely say that much. HUD's layers, for example, define no domains, so analysts still have to find out elsewhere what a field such as HCV_PUBLIC_PCT is a percentage of.

This was the optional last item in thealphacubicle#84 and was deferred from #38.

Proposal

• Fetch the item's metadata document ({portal}/sharing/rest/content/items/{id}/info/metadata/metadata.xml on ArcGIS Online/Enterprise). Do it best-effort and cache it like layer descriptions, so a missing document or a 404 never fails the call.
• Parse it with the standard library xml.etree.ElementTree, with no new dependency. Read the entity/attribute entries (eainfo/detailed/attr: attrlabl, attrdef, attrunit, and rdom/edom where present).
• get_schema adds a Definition: line (and units) per field when the metadata has one, matched by field name. Treat the text as portal content: html_to_text, portal_block, and caps.
• Security: parse with entity expansion off. ElementTree does not resolve external entities, but guard against oversized documents with a size cap before parsing, and consider defusedxml only if its size fits the Lambda budget.
• Tests: an FGDC-style fixture, a missing document, malformed XML, a size cap, and name matching that's case-insensitive.

References

• feat(arcgis): full plain-text descriptions, data dates, source block (#32) #38 (metadata PR), [Feature]: ArcGIS metadata — full plain-text descriptions, data dates, and a source block thealphacubicle/OpenContext#84