Add feature switch to control XmlResolver.IsNetworkingEnabledByDefault

[MichalStrehovsky]

Fixes dotnet/runtime#89877.

[eiriktsarpalis]

The STJ property controlling the System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault swich was named JsonSerializerIsReflectionEnabledByDefault. Echoing that naming convention, shouldn't this also be named XmlResolverIsNetworkingEnabledByDefault?

Typically this needs to go through API review but maybe we can fast-track this. cc @eerhardt @bartonjs @terrajobst

[MichalStrehovsky]

We've not done API review on these or they would be consistent.

The XXXSupport is a prevalent theme for things that disable support for something

DebuggerSupport, EventSourceSupport, HttpActivityPropagationSupport, MetadataUpdaterSupport, NullabilityInfoContextSupport, CustomResourceTypesSupport, DynamicCodeSupport, BuiltInComInteropSupport,...

(The Json one is the odd one out)

[eerhardt]

One difference between the existing XXXSupoort flags and the JsonSerializerIsReflectionEnabledByDefault flag is that the XXXSupport flags completely remove the ability for doing XXX. Where the JsonSerializer is still capable of doing reflection, it just won’t do it by default when the flag is set. Thus the verbose naming (it’s trying to be correct/precise).

I don’t know exactly which case the new XML flag falls into, but if the XmlResolver can still use networking even if the flag is set, I wouldn’t name it XXXSupport. Because it can still do it, even though the flag is set.

[MichalStrehovsky]

@jozkee @krwq do you know which side IsNetworkingEnabledByDefault is on so we can finalize the name?

[eiriktsarpalis]

I'm not familiar with the code, but looking at this change suggests that it falls on the same boat as the STJ switch. It picks the XmlUrlResolver implementation if the switch is turned on, which is a public class. The behavior of XmlUrlResolver itself doesn't seem to be affected by the feature switch.

The choice of name for the feature switch also suggests similarity with the STJ one, so we might want to emulate naming convention for the property as well.

[MichalStrehovsky]

@eiriktsarpalis @eerhardt if this updated name looks good, could you please merge? I'm off work through end of August. It would be nice to have this in 8.0.

[eiriktsarpalis]

Thanks!