Make OpenAPI.NET library trim-compatible

[captainsafia]

This PR removes the use of reflection in GetDisplayName for enums and adds [DynamicallyAccessMembers] attributes to CloneFromCopyConstructor.

Prior to this change, using the Microsoft.OpenApi package in an application with trimming enabled would present the following warnings:

    /_/src/Microsoft.OpenApi/Extensions/EnumExtensions.cs(27): Trim analysis warning IL2075: Microsoft.OpenApi.Extensions.EnumExtensions.GetAttributeOfType<T>(Enum): 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.PublicConstructors', 'DynamicallyAccessedMemberTypes.PublicMethods', 'DynamicallyAccessedMemberTypes.PublicFields', 'DynamicallyAccessedMemberTypes.PublicNestedTypes', 'DynamicallyAccessedMemberTypes.PublicProperties', 'DynamicallyAccessedMemberTypes.PublicEvents' in call to 'System.Type.GetMember(String)'. The return value of method 'System.Object.GetType()' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to.
    /_/src/Microsoft.OpenApi/Any/OpenApiAnyCloneHelper.cs(23): Trim analysis warning IL2075: Microsoft.OpenApi.Any.OpenApiAnyCloneHelper.CloneFromCopyConstructor(IOpenApiAny): 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.PublicConstructors' in call to 'System.Type.GetConstructors()'. The return value of method 'System.Object.GetType()' does not have matching annotations. The source value must declare at least the same requirements as those declared on the target location it is assigned to.

Closes #1715
Closes #1114

[baywet]

I think it'd be interesting to set-up a trimming test project to ensure no regressions happen in the future here.
microsoft/kiota-dotnet#291
microsoft/kiota-dotnet#295
microsoft/kiota-dotnet#296

[captainsafia]

Thanks for the reviews @darrelmiller and @MaggieKimani1 -- I spotted a few more issues after adding a TrimmingTest project (mostly related to ValidationRuleset) so we'll need to hold off on merging this a bit longer.

[captainsafia]

@baywet @eerhardt I think this is good for one last review...

[baywet]

Thank you for this great contribution! And for the ongoing discussion here.

[baywet]

@MaggieKimani1 for final review and merge

[eerhardt]

LGTM. Thanks!

[MaggieKimani1]

LGTM!