Implement Relative Vigor Index (RVI) Strategy

[cinar]

Implement the Relative Vigor Index (RVI) trading strategy using the existing momentum.Rvi indicator.

Location: strategy/momentum/rvi_strategy.go

Strategy Rules:

• Buy Action: RVI crosses above its signal line.
• Sell Action: RVI crosses below its signal line.
• Hold Action: Otherwise.

Implementation Details:

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