Add list shifts for swap request endpoint (#2697)
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"
}
}
]
}
```
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