RESTEasy Reactive - Support text/binary conversion for multipart files

[Sgitario]

Assuming we receive the following request:

Content-Disposition: form-data; name="textPart"; filename="my_file.txt"
Content-Type: application/octet-stream
Content-Transfer-Encoding: binary

We can load the content of the file using the FileUpload class:

public class FormData {
    @FormParam("myFile")
    public FileUpload myFile;
}

However, according to what the specs state:

The presence of the filename parameter does not force an
implementation to write the entity to a separate file. It is
perfectly acceptable for implementations to leave the entity as part
of the normal mail stream unless the user requests otherwise. As a
consequence, the parameter may be used on any MIME entity, even
`inline' ones. These will not normally be written to files, but the
parameter could be used to provide a filename if the receiving user
should choose to write the part to a file.

Then, we should not enforce the use of FileUpload and hence also support reading the whole content of the file into strings, and other binary formats like byte[] and InputStream.

public class FormData {
    @FormParam("myFile")
    @PartType(MediaType.APPLICATION_OCTET_STREAM)
    public String myFile;
}

public class FormData {
    @FormParam("myFile")
    @PartType(MediaType.APPLICATION_OCTET_STREAM)
    public byte[] binaryPart;
}

public class FormData {
    @FormParam("myFile")
    @PartType(MediaType.APPLICATION_OCTET_STREAM)
    public InputStream streamPart;
}

Fix #27083