feat: return tagged variant deserde error

[mattsse]

modifies the derived untagged impl to return the tagged attempt error, which is more useful that the generated "data did not match any variant of untagged enum MaybeTaggedTxEnvelope".

this requires private serde API but this should be okay because this is what untagged expands to.
we could technically make this work if we handroll deserde entirely but would be more error prone

    impl<'de> _serde::Deserialize<'de> for MaybeTaggedTxEnvelope {
            fn deserialize<__D>(
                __deserializer: __D,
            ) -> _serde::__private::Result<Self, __D::Error>
            where
                __D: _serde::Deserializer<'de>,
            {
                let __content = <_serde::__private::de::Content as _serde::Deserialize>::deserialize(
                    __deserializer,
                )?;
                let __deserializer = _serde::__private::de::ContentRefDeserializer::<
                    __D::Error,
                >::new(&__content);
                if let _serde::__private::Ok(__ok)
                    = _serde::__private::Result::map(
                        <TaggedTxEnvelope as _serde::Deserialize>::deserialize(
                            __deserializer,
                        ),
                        MaybeTaggedTxEnvelope::Tagged,
                    ) {
                    return _serde::__private::Ok(__ok);
                }
                if let _serde::__private::Ok(__ok)
                    = _serde::__private::Result::map(
                        <UntaggedLegacy as _serde::Deserialize>::deserialize(
                            __deserializer,
                        ),
                        MaybeTaggedTxEnvelope::Untagged,
                    ) {
                    return _serde::__private::Ok(__ok);
                }
                _serde::__private::Err(
                    _serde::de::Error::custom(
                        "data did not match any variant of untagged enum MaybeTaggedTxEnvelope",
                    ),
                )
            }
        }