Rewrite the Command API

Important: the simplified API now defaults to forwarding interrupts to
subprocesses. I did audit all the call sites, and I think this is a safe change
to make.

- Properly support timeouts with all implementations
- Simplify the API
  - only provide two flavours of blocking calls, which require no input and
    forward interrupts; this is the most common usage
  - provide a number of async calls, which optionally takes input, and a flag
    whether to forward interrupts
  - only support input streams, no byte arrays or other 'convenience features'
    that are rarely needed and unnecessarily increase the surface area
  - use java.time.Duration to specify timeout; for consistency, interpret a
    timeout of <= 0 as no timeout (i.e., including rather than excluding 0)
  - KillableObserver and subclasses are no longer part of the public API, but
    still used to implement timeouts if the Subprocess.Factory does not support
    them
- Update the documentation for Command
- Update all callers; most callers now use the simplified API

PiperOrigin-RevId: 164716782

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