Add missing features to user agent formatting (#865)

* Add missing features to user agent formatter

* Make it possible to set the app name for the user agent

* Add integration test for app name config

* Fix `aws-config` build

* Update changelog

* Create new type for app name to share validation

* Add ability to configure app name programmatically via shared config

* Make app name configurable via profile config and environment variable

* Add functions to add metadata on user agent

* Add breaking change to changelog

* Validate user agent metadata fields

* Add missing lints to `aws-http` and fix them

* Incorporate review feedback