Allow retrieve init and end date of a week given a date

### Overview

Hello, here is the first entry of my proposals for functionalities to import from my current CommonNET project to the Microsoft Community Toolkit, I hope you find it useful (and if not, nothing happens).

In case you need to take as a reference the start or end day of the week in the Gregorian calendar for a given date, also contemplating whether the start of the week is on Monday or Sunday.

This helps to determine ranges for a specific week, for example if you need to do specific logic on a calendar.

For example, I needed it to establish some business rules on a user, for a specific week given a date X.

In the case presented, I'm using DateTime, but of course can be also used from DateTimeOffset and/or DateOnly

### API breakdown

```csharp

namespace CommunityToolkit.Common;

public static class DateTimeExtensions
{
public static DateTime GetFirstWeekDate(this DateTime date, WeekCalendarType weekCalendarType = WeekCalendarType.MondayFirst)
{
//Logic
}

public static DateTime GetLastWeekDate(this DateTime date, WeekCalendarType weekCalendarType = WeekCalendarType.MondayFirst)
{
//Logic
}

}

```

### Usage example

```csharp

var givenDate = new DateTime(2021, 10, 18);

//By default in the proposal, starting on Monday
var initialDay = givenDate.GetFirstWeekDate();
var endDay = givenDate.GetLastWeekDate();

//In this case if week start on Sunday, however the default behaviour can be switched
var initialDay = givenDate.GetFirstWeekDate(WeekCalendarType.SundayFirst);
var endDay = givenDate.GetLastWeekDate(WeekCalendarType.SundayFirst);

```

Examples in current test case file [here](https://github.com/ChustaSoft/CommonNET/blob/main/ChustaSoft.Common.UnitTest/TestServices/DateTimeHelperUnitTest.cs)

### Breaking change?

No

### Alternatives

Add as part of the boilerplate code in requiring project.

No current SDK implementation AFAIK.

### Additional context

This is the first proposal of a series of implementations that I made during time in a common project used for the projects of my OpenSource organization, personal projects and projects of the company in which I work.

Currently, I would prefer to include what is practical in a repository with greater scope and practicality, and eliminate what is not necessary or could be improved, leaving the library in a state of maintenance without new developments.

Please refer to [this](https://github.com/ChustaSoft/commonnet) project

### Help us help you

Yes, I'd like to be assigned to work on this item