Don't allocate the FormFeature eagerly per request

[davidfowl]

Today, in order to pass the form options to the FormFeature, we eagerly allocate it here

HttpAbstractions/src/Microsoft.AspNetCore.Http/HttpContextFactory.cs

Line 49 in 163836f

var formFeature = new FormFeature(httpContext.Request, _formOptions);

. We should find a creative way to avoid doing this so eagerly in the common cases. Here are some options:

• Invent a new way to pass things to features lazily (hard, we tried this and ended up with crazy things)
• Only new up the FormOptions if they were overridden. We'd need a way to detect if the FormOptions were actually set. This is doable but could get ugly as there's no way to detect if options were configured. /cc @HaoK
• Only set the FormFeature if the request is a POST and HasFormContentType. Gross, but would be more pay for play than what we have today.

/cc @vancem @Tratcher

[HaoK]

We could add a new bool property to IOptionsFactory<TOptions> like IsConfigured, so you could then avoid calling IOptions.Value in this case.

[muratg]

@davidfowl Backlogging this until we find a good design to prototype.

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2699