Add playground experiments

[rasca]

Add experiments while at the same time refactoring and improving the stack.

This is the first step onto adding experiments to Flou. From our first users, the feedback we gathered was mostly about adding a description to every Playground run. This prompted us to create the first iteration of Experiments: Playground Experiments where you get to interact with an LTM and iterate over it describing your journey in an Experiment.

The other type of experiment will be datasets trials, which we'll leave for the near feature.

This goes as follows:

• Create Experiments& Trial Entities
  • Automatically assign a name "Entity Experiment N34". Create an index for every experiment and trial
  • Create Playground Trial, choose a better name
    • Creates a new LTM
    • Allow to set trial name
• Add experiment detail
• Add related experiment in playground
• Replace +LTM button with +Experiment
  • Allow a commentary about what happened or what you are going to create
  • Show code diff with the last trial (when we have code as data)
• Show rollbacks in Snapshots Table
  • And the amount of rollbacks from a particular snapshot (and derivates)
  • Show a tree of snapshots & rollbacks, but it's seen from the other side, from the snapshot, not that it roll backs, but that is has N branches from there.
  • Put the last snapshot on top
  • (also show when code changes, what changed)
• Differentiate "Trial by playground" and "Trial by dataset"
• You have inputs and outputs, hypothesis and results / commentaries
  • Each evaluator in a dataset creates an output
  • Add input / outputs to api + studio
    • One or several "notes"
    • You can add a field at any time, it creates it blank in the others (or it doesn't even show them)
    • You can delete fields at anytime
• Experiment analysis
  • You can filter by trial output/input and create custom views and charts
  • You can execute "evaluators" not just in trials but in experiments as well
    • They create outputs

Refactors in this PR

• Share session between pytest & fastapi
• Add superforms to studio
• Improve route handling in studio
• Refactor fastapi into modules each with models, schemas, routes
• Rename executor to engine
• Unify rollback and replay