Support text/plain response as streaming request body (#638)

## Changes
For APIs using the `text/plain` content type (today, only usage
download), support these by simply streaming the response body back to
the end user. This way, they can read the response and save it to a file
or post-process it further.

## Tests
Made a simple app that downloads billable usage to a temporary file:
```go
package main

import (
	"context"
	"fmt"
	"io"
	"io/ioutil"
	"os"

	databricks "github.com/databricks/databricks-sdk-go"
	"github.com/databricks/databricks-sdk-go/logger"
	"github.com/databricks/databricks-sdk-go/service/billing"
)

func writeToTempFile(r io.ReadCloser) (*os.File, error) {
	// Ensure r is closed after reading
	defer r.Close()

	// Create a temporary file
	tmpFile, err := ioutil.TempFile("", "prefix-")
	if err != nil {
		return nil, err
	}

	// Copy data from io.ReadCloser to the temporary file
	_, err = io.Copy(tmpFile, r)
	if err != nil {
		tmpFile.Close() // Close and remove temporary file in case of errors
		os.Remove(tmpFile.Name())
		return nil, err
	}

	// Rewind the temporary file for further reading
	tmpFile.Seek(0, io.SeekStart)

	return tmpFile, nil
}

func main() {
	logger.DefaultLogger = &logger.SimpleLogger{Level: logger.LevelTrace}
	a, err := databricks.NewAccountClient(&databricks.Config{
		Profile:      "...",
		DebugHeaders: true,
	})
	if err != nil {
		panic(err)
	}
	ctx := context.Background()
	res, err := a.BillableUsage.Download(ctx, billing.DownloadRequest{
		StartMonth:   "2023-08",
		EndMonth:     "2023-09",
		PersonalData: true,
	})
	if err != nil {
		panic(err)
	}
	tmp, err := writeToTempFile(res.Contents)
	if err != nil {
		panic(err)
	}
	fmt.Printf("tmp: %v\n", tmp.Name())
}

```
This works.

Note that this test depends on a change to the content type for the
OpenAPI spec for usage download to match the true content type.

openapi/model.go

@@ -299,6 +299,7 @@ type MimeType string
 const (
 	MimeTypeJson        MimeType = "application/json"
 	MimeTypeOctetStream MimeType = "application/octet-stream"
+	MimeTypeTextPlain   MimeType = "text/plain"
 )
 
 // IsByteStream returns true if the body should be modeled as a byte stream.
@@ -311,6 +312,7 @@ func (m MimeType) IsByteStream() bool {
 var allowedMimeTypes = []MimeType{
 	MimeTypeJson,
 	MimeTypeOctetStream,
+	MimeTypeTextPlain,
 }
 
 func (b *Body) MimeTypeAndMediaType() (MimeType, *MediaType) {