Add list shifts for swap request endpoint

[matiasb]

Example request/response:

GET /api/internal/v1/shift_swaps/SSR3FJC9H3HZCHT/shifts

{
	"events": [
		{
			"all_day": false,
			"start": "2023-08-01T00:00:00Z",
			"end": "2023-08-01T03:00:00Z",
			"users": [
				{
					"display_name": "testing",
					"email": "testing",
					"pk": "UWJWIN8MQ1GYL",
					"avatar_full": "http://localhost:3000/avatar/ae2b1fca515949e5d54fb22b8ed95575",
					"swap_request": {
						"pk": "SSR3FJC9H3HZCHT"
					}
				}
			],
			"missing_users": [],
			"priority_level": 1,
			"source": "web",
			"calendar_type": 0,
			"is_empty": false,
			"is_gap": false,
			"is_override": false,
			"shift": {
				"pk": "OK9SS5YP42XRG"
			}
		},
		{
			"all_day": false,
			"start": "2023-08-01T03:00:00Z",
			"end": "2023-08-02T00:00:00Z",
			"users": [
				{
					"display_name": "testing",
					"email": "testing",
					"pk": "UWJWIN8MQ1GYL",
					"avatar_full": "http://localhost:3000/avatar/ae2b1fca515949e5d54fb22b8ed95575",
					"swap_request": {
						"pk": "SSR3FJC9H3HZCHT"
					}
				}
			],
			"missing_users": [],
			"priority_level": 1,
			"source": "web",
			"calendar_type": 0,
			"is_empty": false,
			"is_gap": false,
			"is_override": false,
			"shift": {
				"pk": "OK9SS5YP42XRG"
			}
		}
	]
}