[Feature] barGap can be applied to single series instead of all series

### What problem does this feature solve?

barGap can be applied to create bullet chart.

However, multiple series bullet chart cannot be create because barGap only work for all series instead of single series. In [this issue](https://github.com/apache/echarts/issues/6795), [FirstVertex](https://github.com/FirstVertex) wrote a custom function to create multiple series bullet chart.
It's not convinient for daily work. So, is it possible to make barGap to be applied to single series?

### What does the proposed API look like?

series: [{
name: '2011年',
type: 'bar',
barWidth: 28,
zlevel: 2,
barGap:'30%',
data: [183, 239, 290, 104, 234, 630]
},
{
name: '2012年',
type: 'bar',
barWidth: 40,
barGap: '-120%',
data: [125, 308, 310, 121, 134, 481]
},
{
name: '2013年',
type: 'bar',
barWidth: 40,
barGap: '40%',
data: [125, 308, 310, 121, 134, 481]
},
{
name: '2014年',
type: 'bar',
barWidth: 40,
barGap: '-120%',
data: [125, 308, 310, 121, 134, 481]
}
]

Start by tracing how barGap is interpreted for bar series and whether its current scope is global across the series collection. Use the proposed per-series barGap examples as the acceptance case; done means different bar series can specify independent gaps while existing bar layout behavior remains unchanged.

Written by the indexing model from the issue text.