Bundling Actions Together

**Is your feature request related to a problem? Please describe.**

@gamarin2 asks:

```
Hey! While working with class-based actions, I ended up creating state transitions that must logically be split in two separate actions, but where the second one must always be preceded by the first one.

In my case, it's a FormatSummarizer action that formats messages before transitioning to a Summarizer action. These two actions form what I would call an "action bundle": the second one can't be separated from the first one. For now, I have documented that requirement in my class docstring, but it would be nice to have it more strictly enforced.

I'm sure there are various ways we could achieve this, but perhaps the easiest would be to add a property to the action class where you could specify preceding/following action classes. The AppBuilder would then fail to build if you haven't added the proper transitions.
```

@mdrideout also suggests:
```
I would say that I have a very large number of use cases for this, and creating an entire sub-graph for it is painful.

To piggyback on this, I also have a very large number of use cases for running actions with high concurrency, and have been creating sub-graphs simply for the purpose of running a single action for ~10 items.
but I may be doing this as a workaround because I had a some issues trying to use MappedStates (?) with either pydantic or async.
```

**Describe the solution you'd like**
A way to have an action represent a graph.

```python
class MultiAction:
@abc.abstractmethod
def get_graph() -> Graph:
...

class ActionChain(MultiAction):
@abc.abstractmethod
def get_actions() -> List[[Tuple[str, Union[Action, Callable]]]:
...

def get_graph() -> Graph:
named_actions = self.get_actions()
action_names = [name for name, _ in named_actions]
return GraphBuilder().with_actions(**dict(actions)).with_transitions(zip(action_name, action_names[1:]))
```

**Describe alternatives you've considered**
Could also have `with_subgraph` in builder, similar thing. This allows for packaging at the action level for reusability. Might also want the ability to remap inputs/outputs:

```python
def state_maps() -> Tuple[dict, dict]:
....
```

**Additional context**
Add any other context or screenshots about the feature request here.

Start with AppBuilder and GraphBuilder, then trace how actions, transitions, subgraphs, and MappedStates are represented. Compare the proposed MultiAction and ActionChain entry points with the existing builder APIs, including the with_subgraph alternative. Done means the project has an agreed reusable action-graph design, including any input/output remapping and validation behavior.

Written by the indexing model from the issue text.