feat: a public command channel and container engine access

netOrk reached a host's shell through napalm-linux's private `_send`: an
interactive PTY with stdout and stderr merged and no exit code. For Docker
it went further and opened its own paramiko connections around the driver.

This adds the access layer only, no container model (NetOrk/netork#765):

- `channel.py`: `CommandChannelMixin` declares `run_command()` (stdout,
  stderr, exit code) and `open_stream()` (a `ByteStream` to a running
  command). Declared under TYPE_CHECKING, so hasattr stays truthful.
  `run_on_transport()` and `open_stream_on_transport()` implement both on
  a paramiko exec channel for any SSH driver; `ParamikoExecStream` drains
  stderr on every read so the shared window never stalls.
- `container_engine.py`: `ContainerEngineMixin` with `container_engines()`
  and `open_container_engine()`. The returned `ContainerEngineConnection`
  opens the engine API over `<binary> system dial-stdio` and runs the CLI
  with caller-chosen arguments. The driver decides the binary through
  `_container_engine_binary()`; callers never see the path.
- README: why the container model lives in netOrk and not here.

Additive; the Docker*Dict declarations stay until netOrk no longer reads
them. Version 2.6.0.

Refs #17

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "napalm-device-types"
-version = "2.5.0"
+version = "2.6.0"
 description = "Abstract device-type base classes for NAPALM drivers"
 readme = "README.md"
 requires-python = ">=3.10"