Add support for arbitrary XML in value/extValue

[valkum]

Per RFC 5730 these contain children of <any> and also have <anyAttribute>

Replace the static Undef-only ResultValue with a dynamic ValueElement tree that captures namespace, name, attributes, text, and children from any errValueType content. Update EppResult to support multiple <value> and <extValue> elements (unbounded choice per the spec).

Fixes issues where <epp:undef> is replaced with a concrete object type. See the added tests for examples.

There is a rather new spec that gives more context about the extValue and unhandled namespaces with some examples.
https://datatracker.ietf.org/doc/rfc9038/

I am not entirely sure how to cover the <anyAttribute> on <value>. This for now stores it in a vec with all information from the parser.

The schema is:

<complexType name="resultType">
  <sequence>
    <element name="msg" type="epp:msgType" />
    <choice minOccurs="0" maxOccurs="unbounded">
      <element name="value" type="epp:errValueType" />
      <element name="extValue" type="epp:extErrValueType" />
    </choice>
  </sequence>
  <attribute name="code" type="epp:resultCodeType"
    use="required" />
</complexType>

<complexType name="errValueType" mixed="true">
  <sequence>
    <any namespace="##any" processContents="skip" />
  </sequence>
  <anyAttribute namespace="##any" processContents="skip" />
</complexType>

<complexType name="extErrValueType">
  <sequence>
    <element name="value" type="epp:errValueType" />
    <element name="reason" type="epp:msgType" />
  </sequence>
</complexType>

Zero or more OPTIONAL <value> elements that identify a client-provided element (including XML tag and value) or other information that caused a server error condition.

Zero or more OPTIONAL <extValue> elements that can be used to provide additional error diagnostic information, including:

• A <value> element that identifies a client-provided element(including XML tag and value) that caused a server error condition.

• A <reason> element containing a human-readable message that describes the reason for the error. The language of the response is identified via an OPTIONAL "lang" attribute. If not specified, the default attribute value MUST be "en" (English).

I guess it could make sense to take this ValueElement and use it as a first impl for djc/instant-xml#115 although the hard part is probably implementing serialize for it, due to the missing stack namespace stack.

[djc]

Maybe this ValueElement should live in instant-xml? It doesn't seem EPP-specific in any way.

[valkum]

I agree. I will create a PR with just the ValueElement there.

[djc]

Looks okay!

I think this is semver-breaking? If so, can you add a commit with a semver incompatible version bump (if there isn't one already)?

[valkum]

Cargo.toml is already at 0.5 but that was never pushed to crates.io and there is no git tag.

[djc]

Cool, thanks! Let me know if/when you want a release with this.

[valkum]

Currently driving with a fork. I guess it makes sense to wait with a release until I have everything upstreamed (breaking and non breaking).