Add Channel.CreateUnboundedPrioritized

[stephentoub]

Fixes #62761

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[adamsitnik]

LGTM, thank you @stephentoub

I've found some minor nits, but since you are OOF and this PR needs to be merged for P4 I am going to merge it right away.

[adamsitnik]

nit: this logic could be moved to a helper method to avoid copying the code

private static void ReadNext(Channel<Person> c, int expectedAge, int expectedCount)
{
    Assert.True(c.Reader.TryRead(out p));
    Assert.Equal(expectedAge, p.Age);
    Assert.Equal(expectedCount, c.Reader.Count);
}

[stephentoub]

Thanks, @adamsitnik.