Adding proper types for Links and Images

[daniellockard]

I think this is correct? It should make code easier to write instead of writing something like

Links: []interface{}{
			map[string]string{
				"href": "https://link.to.thing/thing",
				"text": "Link",
			},
		},

[theckman]

@daniellockard to confirm: would this is would result in a compile-time breaking API change?

[daniellockard]

This PR was opened quite some time ago. I haven't actually looked at pagerduty anything in quite some time. I believe it would as it changes the types of images and links to actual types.

[theckman]

@daniellockard thank you for taking the time to comment. That makes sense, and I'll keep this PR on-deck for whenever we look to release v2.0.0. Thank you for the contribution, and I apologize for it taking so long to get some sort of resolution on this.