* Broken InputFormatter into InputFormatter and TextInputFormatter
* Added an exception to ModelState inside ReadAsync on TextInputFormatter
when we can't find a valid encoding to read the body.
- #2722
- make communication of errors from formatters to `BodyModelBinder` explicit
- `JsonInputFormatter` now adds errors to `ModelStateDictionary` with correct key
- change `InputFormatter.SelectCharacterEncoding()` to add an error and return `null` when it fails
- one less `Exception` case and removes some duplicate code
nits:
- improve some doc comments (more `<inheritdoc/>`, `<paramref/>` and `<see/>`)
- add another two `BodyValidationIntegrationTests` tests
This change simplifies InputFormatterContext/OutputFormatterContext by
swapping ActionContext for HttpContext.
This change is important especially for InputFormatterContext as it
decouples ModelState from ActionContext - allowing us to fix a
related bug where the _wrong_ ModelState can be passed in for a
TryUpdateModel operation.