Allowed a private key to use for authentication (FEED_STORAGE_SFTP_PKEY)

[vimagick]

Example Usage:

FEED_STORAGES = {"sftp": "scrapy_feedexporter_sftp.SFTPFeedStorage"}

FEEDS = {
    "sftp://[email protected]/some/path/to/a/file": {
        "format": "jsonlines"
    }
}

FEED_STORAGE_SFTP_PKEY = '''
-----BEGIN RSA PRIVATE KEY-----
...
... base64 encoded ...
...
-----END RSA PRIVATE KEY-----
'''