Avoid a few allocations in StringContent

[MihaZupan]

Same idea as #102859, but for StringContent. Saves the 5 smaller allocations each time.

AFAICT UTF-8 is practically the only encoding used here, with text/plain (default) and application/json being by far the most common media types. This PR special-cases these two combinations.

[Benchmark]
public void Test()
{
    var content = new StringContent("Foo", Encoding.UTF8, "application/json");
    foreach (var header in content.Headers.NonValidated)
    {
        _ = header.Value.ToString();
    }
}

Method	Toolchain	Mean	Error	Ratio	Allocated	Alloc Ratio
Test	main	296.5 ns	2.95 ns	1.00	520 B	1.00
Test	pr	147.2 ns	0.55 ns	0.50	304 B	0.58

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[ManickaP]

Will Header.ContentType be populated in this case?

[MihaZupan]

Yes, behavior-wise as far as the user can see, the two should be the same.

The ContentType property uses the underlying dictionary as the source of truth / storage, and will lazily parse the value when first accessed.