Feature: Order Convenience Functions (order_value, order_percent, order_target)

[MDUYN]

Description

Add high-level ordering convenience functions that simplify portfolio management and rebalancing. The framework currently requires users to calculate order amounts manually.

Desired Capabilities

• order_value(symbol, value): Place an order for a fixed currency amount (automatically calculates quantity)
• order_percent(symbol, percent): Order a percentage of current portfolio value
• order_target(symbol, target_amount): Adjust position to a target number of units (calculates the difference)
• order_target_value(symbol, target_value): Adjust position to a target currency value
• order_target_percent(symbol, target_percent): Adjust position to a target percentage of portfolio

Why This Matters

1. order_target_percent is essential for portfolio rebalancing strategies (e.g., maintain 60/40 allocation)
2. order_value simplifies dollar-cost averaging and fixed-amount strategies
3. order_percent makes it easy to allocate a percentage of capital to each asset
4. These functions reduce boilerplate code and prevent common calculation errors
5. Target-based functions automatically handle existing positions (buy/sell difference)

Current State

The framework has create_limit_order() with percentage_of_portfolio and percentage_of_position parameters, which covers some of these cases. However, there are no target-based functions that automatically compute the difference between current and desired position, and no order_value for fixed currency amounts.

Suggested API

class MyStrategy(TradingStrategy):
    def run_strategy(self, context, data):
        price = data["btc_ticker"]["price"]

        # Rebalance to target allocation — buys or sells to reach target
        context.order_target_percent(
            target_symbol="BTC",
            target_percent=10.0,    # Ensure BTC is 10% of portfolio
            price=price,
            order_type=OrderType.LIMIT
        )

        # Fixed currency amount order
        context.order_value(
            target_symbol="ETH",
            value=500.0,            # Buy 500 EUR worth of ETH
            order_side=OrderSide.BUY,
            price=price
        )

        # Adjust to target position size
        context.order_target(
            target_symbol="BTC",
            target_amount=1.5,      # End up with exactly 1.5 BTC
            price=price
        )

These methods would be added to the Context class alongside the existing create_limit_order() and create_order() methods. Internally they would query current positions and portfolio value, calculate the required order, and delegate to create_order().

[MDUYN]

Implemented in PR #555 (merged to dev at 563a68a). Docs added in 6a99c67. Follow-up MARKET variant tracked in #556.