Allow configuration binder to bind single elements to array

[maryamariyan]

Backport approved by tactics offline for 6.0.

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PR Content

1st PR: #57204
2nd PR: #57872 (applies leftover PR feedback from original)

Customer Impact

When trying to bind from an XML configuration provider, we now can allow to bind single elements to array rather than leaving them as null:

<TvShow>
  <Metadata>
    <Name>Prison Break</Name>
  </Metadata>
</TvShow>

This however would be breaking for other providers (like JSON configuration provider) where this is not expected.
So for example, when binding to:

public class TvShow
{
  public Metadata[] Metadata { get; set; }
}

public class Metadata
{
  public string Name { get; set; }
}

Even if we change the below JSON:

{
   "tvshow": {
        "metadata": [
            {
               "name": "PrisonBreak"
            }
         ]
    }
}

so that tvshows is an object with a single metadata:

{
   "tvshow": {
        "metadata": {
            "name": "PrisonBreak"
          }
    }
}

would still bind properly even though json is not specifying an array.

Testing

Using the test that is part of the linked PRs.

Risk

Low – Might incur a little bit of perf cost when app context switch is set to true.

If a customer is not interested in this breaking change they can set the introduced app context switch Microsoft.Extensions.Configuration.BindSingleElementsToArray to false and get the old behavior back.

Regression?

No

More links

Visit dotnet/docs#25779 for more information.

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Issue Details

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Approved by tactics offline.

Author:	maryamariyan
Assignees:	-
Labels:	area-Extensions-Configuration
Milestone:	-

[ericstj]

Approved by @SteveMCarroll in mail.