Allow formatting of SSH keys

[bdsoha]

Summary

The current implementation does not allow retrieving SSH private keys in open-ssh format.
When using the CLI and the read subcommand, I can provide ?ssh-format=openssh to the end of the path to return the key in open-ssh format.

Proposed solution

Allowing query parameters to be appended to lookup keys, when using property

func (provider *ProviderOnePassword) getField(item *onepassword.Item, property string) ([]byte, error) {}