This PR implements the feature requested in [#3658] to allow configurable telemetry for the Client.

Modifications -

• WithCallback(callback func()) was added. The callback will be set by the user.
• The callback execute just before the client.bareDo inside client.Do.
• Allows for chaining because it returns the same Client instance.
• Added new Callback field in the Client struct of type func(), which default set to nil during initialize().

Example Code -

client := github.NewClient(nil).WithCallback(func() {
fmt.Println("Callback executed")
})

This enables users to attach custom telemetry or logging kind of things to API calls in a flexible, user friendly manner.
Btw this is my first PR
Happy to get any feedback or suggestions to improve it