civil: add AddMonths, AddYears and Weekday methods to Date, DateTime

[Vinayaka-B]

Is your feature request related to a problem? Please describe.

While using Date and DateTime objects, there might be need for certain operations like -

1. Getting the date n months in future from given date
2. Getting the day of the week for a certain date

Describe the solution you'd like

Implementing the methods for the structs

Describe alternatives you've considered

User needs to convert object in time.Time struct do the above operation and convert back to the Date struct. That is inconvenient.

Additional context

Will create a PR for this soon

[Vinayaka-B]

Avoided adding the methods for DateTime struct due to this comment in the code -

// Note: We deliberately do not embed Date into DateTime, to avoid promoting AddDays and Sub.

Don't fully understand why we need to avoid these methods (AddDays, AddMonths, Weekday) for the struct so I can update my PR if some maintainer can confirm that it is okay to add them

[quartzmo]

@Vinayaka-B Thank you for suggesting these additions.

there might be need for certain operations like...

What is your immediate motivation for adding these functions now?

[Vinayaka-B]

I was writing an application where I needed the above. I had to convert the Date struct object to time.Time object and then convert back to Date. While that is a working solution, I feel the problem is generic enough that this should be supported directly by the package.

[quartzmo]

Closed by #11340