support set ping handler and pong handler

[qcha0]

first of all, thanks this good project, we are preparing a tool based on this project.

Sometimes i need do something while receive ping or pong message.
So I added this feature, is it alright

[nhooyr]

Thanks for implementing this @qcha0

Related issue is #246

What's your usecase out of curiosity for needing to instrument ping/pongs?

[qcha0]

just like i want to count the number of ping，or set close the connection if there is no ping message whithin two cycle.

[schmidtw]

@nhooyr I need pings/pongs at the application layer for an app we're building a replacement for. I know work is being done on the dev branch, but adding this one feature to a trusted, working code base prevents me from needing to fork the repo for just this one feature we're trying to ship in a week.

Could this PR be merged?

[nhooyr]

@schmidtw Sorry I won't get to this for a few weeks still. There's another change I wanted to make to let people handle close frames too. What feature are you trying to ship that relies on this?

[schmidtw]

I have a protocol that depends on sending & receiving websocket pings and pongs. Each side of the connection expect a ping/pong within an interval & if either doesn't get the ping or pong from the other side the connection is determined to be broken & application logic notes the reason, closes down immediately & the client side will reconnect. The pings and pongs are reported as metrics as well. This is the project: https://xmidt.io/

Since this is weeks out & behind other PRs and the dev branch, it makes sense for me to copy the code, apply this patch & use it locally in my project. When these features are available upstream I can delete the code from the project & move back to this code. I'm not interested in staying on a forked copy for any longer than is needed.

[nhooyr]

Makes sense. Yes, please fork the code for now.