[Linq] Implement allowMultiple parameter on SingleOrDefault

[Tim-Maes]

See #119209

SingleOrDefault would have been the best name for this, but since it is already taken, I propose this overload.

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[Tim-Maes]

@dotnet-policy-service agree

[eiriktsarpalis]

Unfortunately, we cannot accept any PRs introducing new APIs unless those APIs have already been approved via our API review process. I would suggest creating a new issue as an API proposal first, after which we can consider the possibility of us accepting this.

Until then, I am going to close this PR. Thanks!