Fix S6580 FP/FN: Do not raise when DateTimeStyles.RoundtripKind is specified

[gregory-paidis-sonarsource]

Description

When using one of the following methods with the overload that accepts a DateTimeStyles argument and DateTimeStyles.RoundtripKind is used, a format provider does not need to be specified.

Repro steps

using System.Globalization;

var before = new DateTimeOffset(2008, 4, 12, 9, 30, 0, new TimeSpan(-8, 0, 0));
// converted to string with the "o" ISO round-trip-style. DateTimeKind is "baked" in the string.
var stringified = before.ToString("o"); 
// RoundTripKind is enough to parse this.
var after = DateTimeOffset.Parse(stringified, null, DateTimeStyles.RoundtripKind); // Noncompliant FP
Console.WriteLine("Round-tripped {0} to {1}.", before, after); // before == after

You can actually see this on Microsoft's msdn code snippet about the "o", ISO-roundtrip style format (click me)

Applies to overloads of these methods:

DateTime.Parse
DateTimeOffset.Parse
DateTime.TryParse
DateTimeOffset.TryParse
DateTime.ParseExact
DateTimeOffset.ParseExact
DateTime.TryParseExact
DateTimeOffset.TryParseExact

[BennieCopeland]

In addition to this, I don't think it should raise when I provide the expected format/s:

var before = new DateOnly(2008, 4, 12);
var stringified = before.ToString("o");
// Unfortunately, RoundTripKind doesn't work with DateOnly
var dt = DateOnly.ParseExact(stringified, "yyyy-MM-dd")