Missing Convert.ToBase64 with Base64FormattingOptions in .NET Standard 1.6 and lower

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https://docs.microsoft.com/en-us/dotnet/api/system.convert.tobase64string?view=netframework-4.7.2

ToBase64String(Byte[], Int32, Int32, Base64FormattingOptions) ToBase64String(Byte[], Base64FormattingOptions)

https://docs.microsoft.com/en-us/dotnet/api/system.convert.tobase64chararray?view=netframework-4.7.2

ToBase64CharArray(Byte[], Int32, Int32, Char[], Int32, Base64FormattingOptions)