[csharp-netcore] add options dateFormat, dateTimeFormat to customize the format

[devhl-labs]

Enables users to use a template to set the default formats of DateTime and Date.

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[wing328]

FYI. In the python-nextgen generator, we added 2 options to customize the datetime, date form: datetimeFormat, dateFormat.

[wing328]

FYI. I've filed #14932 update the description a bit.