Move IServerVariables feature and friends to Http.Abstractions

The IIS server adds an HTTP feature IServerVariablesFeature. Server Variables are not an IIS-exclusive concept and date all the way back to CGI so it seems reasonable that a server may want to provide them. We should move this type out of Servers.IIS and in to HttpAbstractions.

Specific changes:

• Move IServerVariablesFeature to Http.Features
• Add GetServerVariables extension method in Http.Features.
• Change GetIISServerVariables extension method to just call GetServerVariables (and mark as [Obsolete]?)
• Update all reference assemblies
• Add TypeForwardedToAttribute to Servers.IIS’s AssemblyInfo.cs pointing at the new type.

[Tratcher]

Type forwarding IServerVariablesFeature would be compatible, no? Unless you change the namespace.

GetIISServerVariables can stay for compat, GetServerVariables would be a new extension method. Note GetServerVariables does not go in Http.Features, it would go into Http.Extensions.

[jkotalik]

Acceptance checklist (check one item)

• We decided not to take this fix.
• The fix is tests-only.
• The fix contains product changes (check all items below).
  • Relevant XML documentation comments for new public APIs are present.
  • Narrative docs (docs.microsoft.com) are updated. (check one item below)
    • The change requires a new article. An issue with an outline has been filed here: [ISSUE LINK]
    • The change requires a change to an existing article. A docs PR with these changes is linked here: [PR LINK]
    • The change requires no docs changes.
  • Verification has been completed. (check one item below)
    • Verified against ASP.NET Core App Version 3.0.0-preview7.19351.2