ssh: Add extensions flag to ssh

[kirill-aleksandrov]

#5659

Currently, ssh -mount-point=some_ssh -role=some_role -mode=ca user@host tries to use all possible extensions, even if some are not allowed by the role, and there is no way to provide a list of extensions.

Added flag -extensions accepting a list of ones. I've tested that with an empty list the role's default extensions are used.

Important: Potentially, it's a breaking change since not all extensions might be used by default

[hashicorp-cla]

All committers have signed the CLA.

[gardar]

Any chance this will be merged soon?