Allow to set the color of markLine symbols

### What problem does this feature solve?
Currently it is not possible to set the color of the symbols of a markLine (different to the line-color)


### What does the proposed API look like?
In addition to setting the symbol, we could add a `symbolColor`, e.g.

```
markLine: {
data: [
[{
name: 'start',
coord: [0, 50],
lineStyle: {
color: 'blue'
},
symbol: 'rect'
symbolColor: 'blue'
}, {
name: 'end',
coord: [1, 500],
symbol: 'rect'
}]
]
}
```

* Reference to [MarkLine API docs](https://echarts.apache.org/en/option.html#series-line.markLine.data.0.symbol)
* related [Stackoverflow question: How to change the symbol color of a mark line?](https://stackoverflow.com/questions/66102322/how-to-change-the-symbol-color-of-a-mark-line)

Start with the MarkLine API documentation, especially the series-line.markLine.data.0.symbol entry and the proposed markLine configuration in the issue. Trace how markLine symbols and lineStyle colors are handled, then verify that symbol color can be configured independently and that the documented API behavior is covered by tests.

Written by the indexing model from the issue text.