Support avg function in WindowStream

[ni-ze]

[Jargon9]

Hi teams, I want to handle this. Could you please assign it to me ?

[ni-ze]

@Jargon9 OK, It already assign to you. Please clone from develop branch, and merge the your pr back into it.
The develop branch is the default branch, I will change the default branch in the next feature days.

[Jargon9]

@ni-ze Got it! thanks very much.

[Jargon9]

Hi teams, I have completed the main part of avg calculation. Could you please review the code?