Add support for multiparat (MIME) user data for Linux environments. This
type is more versatile type of user data, and some AWS service
(i.e. AWS Batch) requires it in order to customize the launch
behaviour.

Change was tested in integ environment to check if all
user data parts has been executed correctly and with proper charset
encoding.

fixes aws#8315

* Remove `IMultipartUserDataPartProducer`
* Add `MultipartUserDataPart` & `IMultipart`
* Concrete types to represent raw part and UserData wrapper can be created with
`MultipartUserDataPart.fromUserData` & `MultipartUserDataPart.fromRawBody`
* Removed auto-generation of separator (as with tokens hash codes can differ when tokens are not resolved)

- remove `MultipartContentType`
- remove `MultipartUserDataPartWrapperOptions`
- remove `IMultipart`
- rename `MultipartUserDataPart` -> `MultipartBody`
- other removals
- restructure other classes
- moved part rendering to part class
- set default separator to hard codeded string
- added validation of boundry