Convert VALIDATOR_TEST_CATEGORY to enum

[petrsvihlik]

Motivation

• Utilize strong types

Describe the solution you'd like https://github.com/petrsvihlik/WopiHost/blob/468ca43e3ebc6093cb35d63ef697fc68126a46d2/WopiHost.Url/WopiUrlSettings.cs#L155

• According to the docs the parameter can be of the following values: All, OfficeOnline, or OfficeNativeClient.
• Create a corresponding enum within the WopiHost.Url project (it should be named ValidatorTestCategoryEnum)
• Convert the type of the ValidatorTestCategory property in WopiUrlSettings from string to the new enum
• Add a test in WopiHost.Url.Tests to verify the de/serialization works fine