Upgrade to .net core 3.1

[XeNz]

Currently this library is incompatible with .NET Core 3.0 and upwards.

I would like to suggest an upgrade to support .NET Core 3.1 projects.
This would entail:

• Upgrading any Microsoft.AspNetCore package to 3.1.0 if possible
• Adding the new Newtonsoft package
• Resolving MvcOptions issues
• Rewriting IUrlHelper DI, this has also changed since 3.0
• Upgrading the Swashbuckle library to 5.0.0-rc5. This holds many breaking changes and will also result into Swagger OperationFilters and DocumentFilters to be rewritten.
• Updating the sample project to accomodate these changes

Why?

.NET Core 2.2 will reach End of Life on December 23, 2019. This means it's a good idea for projects to get upgraded to 3.0 and onwards.

Created a PR as starting point:

#33

Currently broken:

• Versioning
• Usage of MvcOptions, as expected.

Questionmarks:

• ValidRefUri filter
• Need a workaround for setting Consumes and Produces

[XeNz]

Update on PR:

• FIxed API versioning and MvcOptions usage
• Consumes and produces still broken : https://github.com/digipolisantwerp/web_aspnetcore/blob/d9e70144cc6e8a2e5add81063aed4f93c5a9d8f8/src/Digipolis.Web/Startup/MvcBuilderExtensions.cs#L82-L92 is not usable since JsonOutputFormatter is not in the same package anymore. I guess manual enforcement of Consumes and Produces attributes could be done?
• Unsure if ValidRefUri.cs is still needed