[SSHD-758] Add OpenSSHGeneratorFileKeyProvider

[cschneider]

[lgoldstein]

As indicated in the issue we don't intend to use this code in the foreseeable future. Even if we were, it is a long way from being up to the project's standards - especially lacking full coverage of all possible keys - not just RSA - and all possible PEM formats (there are quite a few of them...). Furthermore, the issue indicates a desire to not use Bouncycastle but simply replaces it with another library - so I fail to see to difference...

[tomaswolf]

Closing old outdated PRs.