GROOVY-8901: Add DGSM countDistinct[By], sum[By], avg[By], etc.

[daniellansun]

No description provided.

[paulk-asert]

Time to write some adoc doco for these new collector methods? ;-)

I'll try to have a bit of a better look on the plane flight to Java2days. I guess we should have avg for non-streams too. I also wonder whether there are any naming alterations needed to make stream and non-stream versions align? I'll go through some examples and let you know.

[daniellansun]

I'll try to set aside some to write some doco.