Make `MarketSimulator`, `TransactionCost`, `HoldingCost` agnostic to asset class and subclass them to specialize them

[enzbus]

Introduce classes such as StockMarketSimulator, StockTransactionCost, ..., that specialize the parameters to a given asset class. Other ones could be FuturesMarketSimulator (still using YFinance), HousingMarketSimulator (using Case-Shiller via FRED), ...

The general classes would have little to no default parameters. Subclasses would only override the __init__, essentially to make it easier for users (because the full list of parameters could be difficult to understand).

Rationale

• original interface had no default parameters and costs specialized to stocks (per_share_cost, spread_on_borrowing_stocks_percent, ..., were introduced in 2023)
• it's better design to have a general interface that can be specialized, rather than a special one that can be generalized
• the specialized ones (i.e., the current ones) should be kept, in order to make the examples more terse; important logic in the examples should be avoided
• separate the logic of general models (e.g., transaction cost) from selection of accurate parameters - both are important, conflating them makes the code too heavy
• specialized classes would be very little code and easy to maintain; each one should get an example (same applies to policies and constraints!)