refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing - #492

Use generic type parameters to express that SSHResult and LocalCommandResult
return specific output types (str or bytes) based on the decode parameter,
removing the need for isinstance checks in calling code.

Signed-off-by: Gaëtan Lehmann gaetan.lehmann@vates.tech

This PR is part of a complex set of PRs containing 6 PRs:

1. master
2. Use passlib in place of legacycrypt #396 → master
3. Introduce TOML-based configuration to replace data.py #491 → Use passlib in place of legacycrypt #396
4. "refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing" (this PR) → master
5. misc type related enhancements #460 → refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing #492
6. Add ty type checker configuration #476 → misc type related enhancements #460
7. Add Docker-based compatibility test kit for external validation #431 → refactor: make BaseCmdResult and subclasses generic to eliminate runtime type narrowing #492, Use passlib in place of legacycrypt #396