Implement the pkg/ssh.AuthInterface. Add logic for converting from the
ssh stream state to an evaluator request, and for interpreting the
results of policy evaluation. Refactor some of the existing authorize
logic to make it easier to reuse.
## Summary
This implements the StreamManagement API defined at
https://github.com/pomerium/envoy-custom/blob/main/api/extensions/filters/network/ssh/ssh.proto#L46-L60.
Policy evaluation and authorization logic is stubbed out here, and
implemented in https://github.com/pomerium/pomerium/pull/5665.
## Related issues
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- #159
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## User Explanation
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change doesn't create any user-facing changes, you can leave
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## Checklist
- [ ] reference any related issues
- [ ] updated unit tests
- [ ] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [ ] ready for review
