Add a new `TestcontainersStartup.start` static method and update the
existing start methods so that `Startable.start()` is only called when
the container is not already running.
Prior to this commit, we assumed that `Startable.start()` calls were
idempotent and could be safely made multiple times. Whilst this appears
to be true for stock `GenericContainer` based startables, users may have
their own `start()` method that does not expect to be called multiple
times.
The implemented detection logic will not be applied if a `Startable`
is not also a `Container`. In these cases, the implementation will need
to deal directly with multiple `start()` calls.
Fixed gh-43253
ccc1b5da28