- #549, #592
- move feature implementations to `Microsoft.AspNetCore.Http.Features` and `...Authentication.Features`
- move `DefaultHttpContext`, `HttpContextAccessor`, `HttpContextFactory`, `FormCollection` and
`HeaderDictionary` to `Microsoft.AspNetCore.Http`
- move `FormFile` to `Microsoft.AspNetCore.Http.Internal`
- that and `Microsoft.AspNetCore.Http.Authentication.Internal` are the remaining `.Internal` namespaces
nits:
- remove a couple of parameterless constructors
- add / fill subfolders to align with new namespaces
- remove all use of (unnecessary) "T:..." `<see cref=""/>` values
- #561
- new `SetCookieHeaderValue.AppendToStringBuilder()` method; avoids per-call `StringBuilder` allocation
- `ResponseCookies` uses `ObjectPool<StringBuilder>` that `ResponseCookiesFeature` provides
- `ResponseCookies` works fine if no `ObjectPoolProvider` is available
- `IHttpContextFactory` instance is a singleton instantiated from CI
- make `HttpContextFactory` `ObjectPoolProvider` and `ResponseCookiesFeature`-aware
- apply same pattern to sample `PooledHttpContextFactory`
- pool is not currently configurable; defaults are fine for response cookies
- if we need (policy) configuration, would add an `IOptions<HttpContextFactorySettings>`
nit: Add some doc comments