Implement Donchian Channel Breakout Strategy

[cinar]

Implement the Donchian Channel breakout trading strategy using the existing volatility.DonchianChannel indicator.

Location: strategy/volatility/donchian_channel_strategy.go

Strategy Rules:

• Buy Action: Current price is above the Upper Donchian Channel (e.g., 20-period high).
• Sell Action: Current price is below the Lower Donchian Channel (e.g., 20-period low).
• Hold Action: Otherwise.

Implementation Details:

• Follow the strategy.Strategy interface.
• Use volatility.NewDonchianChannel() or allow custom parameters.
• Provide unit tests with historical data in strategy/volatility/testdata/donchian_channel_strategy.csv.
• Ensure 100% test coverage.