Add missing `Endpoints` and `Functions`

[AndreasReitberger]

Add missing Endpoints and Functions to the library

• [x] Contact.Add
• [x] Contact.Update
• [x] Countries
• [x] Payments https://github.com/AndreasReitberger/LexOfficeClientSharp/pull/30

[AlexHolly]

Is there a reason for using double instead of decimal, for example in LexQuotationTotalPrice.cs?

It is generally recommended to use decimal in financial systems to avoid precision issues.

[AndreasReitberger]

No, I'm just used to use double in my classes. I will switch to decimal then ;) Thanks for the hint!

Edit: Done in https://github.com/AndreasReitberger/LexOfficeClientSharp/pull/38/commits/8e22148da510050714c36419bbcbcc78b33ffe65