Add support for SSH private key passphrase

[MichaelKoster70]

As a developer I want to protect my SSH private key with a passphrase.
The extension shall detect that the private key is protected and ask for the passphrase.

• The shall be cached for the time the visual studio solution is loaded.
• The passphrase shall be kept secure while in memory

[MichaelKoster70]

Postpone until we have a custom SSH adapter that can handle passphrases in a proper wa<