Add support for downloading a DB Backup via the API

[typhonius]

https://cloudapi-docs.acquia.com/#/Environments/getEnvironmentsDatabaseDownloadBackup

[typhonius]

We can either pass this directly back as a Guzzle Stream object, or convert it to a string. To do the latter, we will have to rewind() and then getContents() the Stream response object.

I think I have a small preference to pass back a string rather than an object, although really we should implement the method consumers of the SDK would expect.

[typhonius]

Mock code:

DatabaseBackups.php

    public function download($environmentUuid, $dbName, $backupId)
    {
        return $this->client->requestStream(
            'get',
            "/environments/${environmentUuid}/databases/${dbName}/backups/${backupId}/actions/download"
        );
    }

Client.php

    public function requestStream(string $verb, string $path, array $options = [])
    {
        $response = $this->makeRequest($verb, $path, $options);

        try {
            $stream = $this->processResponse($response);
            $stream->rewind();
            return $stream->getContents();
        } catch (ApiErrorException $e) {
            exit($e->getErrorType() . ' error: ' .  $e->getMessage());
        }
    }

[typhonius]

Looking at what has been done with drush aliases and log downloads, my initial thought was incorrect and I should be passing the StreamInterface object back for consumers to work with.

This is the approach taken in #40