Add workflow graph generation functionality

[oEscal]

This pull request implements feature #29.
It uses the Python transitions library, creating a state machine representing the given workflow. From that state machine, it can generate a graph using graphviz or the corresponding Mermaid code (as with the TypeScript SDK). By generating a state machine, it gives more flexibility to programmers to have more observability into the workflow itself programmatically.
Apart from generating a state machine and the corresponding graph, this implementation adds new features in relation to the TypeScript SDK one:

• If the developer decides to do so, it also gets all the functions and their transitions related to actions inside a state. Therefore, the developer can create a graph that not only represents the workflow states and their interconnections, but also the actions and transitions of each state;
• It can build the state machine and corresponding graph for workflows with subflows. The subflow will be generated as a nested state of the corresponding state/action.

[cdavernas]

LGTM!

[cdavernas]

LGTM! Cheers ❤️