MWI: Decouple identity output for SSH/Identity use-cases

[strideynet]

Today, when you want to generate an identity file for use with an access plugin or API client, you use the identity output of tbot. When you want to generate a SSH config, you use the identity output of tbot. Effectively, the identity output is overloaded for multiple use-cases.

This has a few negative impacts:

• Unnecessary work for a use-case is completed (e.g generating SSH configs) and a failure of this unnecessary work can prevent the output generating correctly.
• Overly complex UX, users must understand which configuration options are relevant to their use-case, and these two use-cases contrast significantly.

We should create a plan to decouple the UX for these two stories. We'll want to avoid creating any breaking changes. As an initial thought for discussion:

1. Introduce identity/v2 which is designed for pure identity use-cases
2. Introduce openssh which is designed to provide an OpenSSH compatible configuration/credentials
3. Provide a migration command which moves users from identity to openssh or identity/v2 depending on their usecase.