Implement `price()` and `price_history()`

[AlexTheWizardL]

This issue covers creating price() and price_history() methods that are described in the abstract class

General understanding:

In previous meetings, we've decided that add_price() and delete_price methods are not available in the cashctrl system (this is now mentioned in the milestone description).

In the Standalone_Ledger, these prices are stored as a class field called _prices. Price entity can contain ticker, currency, price, and date fields.

Ticker is a concept that can be represented by any name and has some price. For example ticker - cow can cost 100 USD, ticker cat can cost 10 UAH. Also, that price can vary in time, for example in 2024 cow costs 100 USD but in 2025 it costs 200 USD

Following questions:

1. Where price along with ticker should be stored in the scope of Cashctrl_ledger?
2. If the add_price() method is not available in the cashctrl system then what sense of get methods if we can not create a price?
3. Is there any corresponding entity that exists in the cashctrl system?

[AlexTheWizardL]

Proposed solution and understanding:

• Ticker in this issue scope would be currencies
• Price would return just exchange rate
• Interface for price(currency: str, date: date.datetime) -> float; would return just one value via API call for exchange rate
• Interface for price_history(currency: str) -> pd.DataFrame, would return df with a price and date columns that will correspond to price change along the history

[AlexTheWizardL]

After trying to implement this issue I've discovered new pitfalls of the Cashctrl API:

• There is no way to receive a history of price changes. The "currency/exchangerate" endpoint is the only one to calculate the exchange rate and it returns only one float value at a time
• My goal was to test an exchange rate for a created currency. After creating a custom currency using the currency/create.json endpoint and trying to receive its rate from the "currency/exchangerate" endpoint it returns an empty string and doesn't see a new currency