Add ability to define event properties across all events

[jvitoroc]

This is going to be useful for me because I have several Go services, and I want to differentiate between them in the dashboard insights.

posthogClient, err = posthog.NewWithConfig(
		"phc_",
		posthog.Config{
			BatchSize:  1,
			Endpoint:  "https://us.i.posthog.com",
			...
			DefaultEventProperties: posthog.NewProperties().Set("service", "api"),
		},
	)

[dmarticus]

Nice work! Thank you for your contributions!