Add asset class, envelope, expected return, and investment time

[MadeInPierre]

To generate analytics, it would be nice to specify several attributes on each Line, including :

• [ ] Asset class with an enumeration AssetClass.STOCKS
• [ ] Expected return (yearly returns for now, we'll see about simulation scenarios later)
• [ ] Custom currency symbol
• [ ] Recommended investment time
• [ ] Define envelopes before the portfolio. Finalynx could provide pre-made common envelopes like PEA, PER, AV, ... and add availability information (we'll see about taxes later):

  Envelope("PEA", time_blocked=5)
  Envelope("AV",  time_blocked=0, time_taxed=8, unblock_events=["retirement, buy_home"])

In the end, a Line definition could look like this:

pea = Envelope("PEA", date_created=date(2023, 03, 21), ...)
return_stocks = Return(min=-2, mid=6, max=10, time_recommended=timedelta(years=8))

portfolio = Portfolio(children=[
    Line("ETF World", key="...", envelope=pea, class=AssetClass.STOCK, return=return_stocks)
])

Other ideas are welcome!