feat: Refactor CLI config-ssh to improve UX (#1900)

- Magic block is replaced by Include statement
- Writes are only done on changes
- Inform user of changes via prompt
- Allow displaying changes via `--diff`
- Remove magic block if present
- Safer config writing via tmp-file + rename
- Parse previous `config-ssh` options, compare to new options and ask to use new (otherwise old ones are used)
- Tests the new functionality

Fixes #1326