Feature Request: Add Trade Signal Support for Candlestick Charts #1995

Is your feature request relasted to a problem? Please describe.
I'm developing a financial application that displays candlestick charts for stock/crypto
trading data. After showing the K-line chart, I need to display user's buy/sell transaction
records as signal markers on the chart.
Currently, fl_chart doesn't support rendering trade signals directly

Describe the solution you'd like
I would like fl_chart to provide a simple interface where I can pass in trade signals and they
will be directly displayed as buy/sell markers on the candlestick chart.

Describe alternatives you've considered
I have considered implementing this by passing in a custom candlestickPainter. However, I
believe that for financial K-line charts, buy/sell signals are a common requirement, so it
would be best if this functionality could be built-in.

Additional context
Here is my PR link #1996
and achieve results