v2: Make it easier to model OpenAPI tags as a hash set

[captainsafia]

The OpenAPI specification states that:

Each tag name in the list MUST be unique.

To reflect this attribute, it would be great if we could model the top-level tags collection in to the document as a hash set of sorts. However, because it is currently modeled as an IList it is difficult to encode this uniqueness constraint in the source:

OpenAPI.NET/src/Microsoft.OpenApi/Models/OpenApiDocument.cs

Line 74 in 47bf1eb

public IList<OpenApiTag>? Tags { get; set; } = new List<OpenApiTag>();

We should consider taking a breaking change to change the type to ICollectionor IEnumerable so that consumer can initialize a HashSet to enforce the uniqueness constraint.

[captainsafia]

I should also add that this will require some sort of comparer implementation for OpenApiTag type. Here's one that we implement currently in ASP.NET Core.

https://github.com/dotnet/aspnetcore/blob/f42307ccd08ee02c885de9427acaf3c8f37fc377/src/OpenApi/src/Comparers/OpenApiTagComparer.cs