Support `mean` as synonym for `avg`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
This came up in datafusion-contrib/datafusion-python#56

Describe the solution you'd like
Support "mean" as well as "avg"

Describe alternatives you've considered
None

Additional context
None

[andygrove]

If anyone is interested in picking this up, it looks like it is probably a one liner (plus tests).

We need to add an additional mapping here in datafusion/expr/src/aggregate_function.rs

impl FromStr for AggregateFunction {
    type Err = DataFusionError;
    fn from_str(name: &str) -> Result<AggregateFunction> {
        Ok(match name {
            "min" => AggregateFunction::Min,
            "max" => AggregateFunction::Max,
            "count" => AggregateFunction::Count,
            "avg" => AggregateFunction::Avg,

[BaymaxHWY]

I'm interested in this. May i try it?