Implement Hitobito event participants endpoint

The get event participants endpoint allows eCamp users to retrieve leaders and co-leaders of a Hitobito event they have access to.

```jsonc
// GET /api/hitobito//events//participants

// Response
{
"_links": {
"self": {
"href": "/api/hitobito/pbsmidata/events/123/participants"
}
},
"totalItems": 3,
"_embedded": {
"items": [
{
"firstName": "Ellen",
"lastName": "Bloch",
"nickname": "Quo",
"email": "bloch.ellen@hitobito.example.com"
},
{
"firstname": "Lee",
"legalName": "Frauen",
"nickname": "Maiores",
"email": "frauen_lee@hitobito.example.com"
},
{
"firstname": "Cindy",
"legalName": "Schwalm",
"nickname": "Et",
"email": "schwalm_cindy@hitobito.example.com"
}
]
}
}
```

## Implementation

1. Retrieve / verify access token
- See https://github.com/ecamp/ecamp3/issues/10400
2. Check that the user has access to the specified event
- See https://github.com/ecamp/ecamp3/issues/10419
3. Retrieve all event participants from Hitobito

```
GET /api/event_participations
?include=roles,participant
&filter[event_id][eq]=
&fields[event_participations]=active
&fields[people]=first_name,last_name,nickname,email
&page[number]=
&page[size]=20
```
Example Response
```jsonc
{
"data": [
{
"id": "20",
"type": "event_participations",
"attributes": {
"active": true,
},
"relationships": {
// ...
"participant": {
"data": {
"type": "people",
"id": "119"
}
},
"roles": {
"data": [
{
"type": "event_roles",
"id": "20"
}
]
}
}
}
],
"included": [
{
"id": "119",
"type": "people",
"attributes": {
"first_name": "Edgar",
"last_name": "Falter",
"nickname": "Facere",
"email": "edgar_falter@hitobito.example.com"
},
"relationships": { /* ... */ }
},
{
"id": "20",
"type": "event_roles",
"attributes": {
"participation_id": 20,
"type": "Event::Role::Participant",
"label": null
},
"relationships": { /* .. */ }
}
],
"links": {
// ...
"next": "/api/event_participations?fields%5Bpeople%5D=first_name%2Clast_name%2Cnickname%2Cemail&filter%5Bevent_id%5D%5Beq%5D=2&include=roles%2Cparticipant&page%5Bnumber%5D=2&page%5Bsize%5D20" // not set if there are no more pages
},
"meta": {}
}
```

7. For every retrieved page, perform the following steps:
1. Go through each participation
2. Check whether **one of** the corresponding event roles has one of the following `attributes.type` (person is co-leader or leader)
- `Event::Role::Leader`
- `Event::Role::AssistantLeader`
- `Event::Role::Cook`
- `Event::Role::Helper`
- `Event::Role::Treasurer`
- `Event::Role::Speaker`
- `Event::Course::Role::Leader`
- `Event::Course::Role::ClassLeader`
- `Event::Course::Role::Advisor`
- `Event::Course::Role::Helper`
- `Event::Camp::Role::AssistantLeader`
- `Event::Camp::Role::Helper`
- `Event::Camp::Role::LeaderMountainSecurity`
- `Event::Camp::Role::LeaderSnowSecurity`
- `Event::Camp::Role::LeaderWaterSecurity`
- `Event::Camp::Role::Leader`
- `Event::Camp::Role::Abteilungsleitung`
- `Event::Camp::Role::Coach`
- `Event::Camp::Role::AdvisorMountainSecurity`
- `Event::Camp::Role::AdvisorSnowSecurity`
- `Event::Camp::Role::AdvisorWaterSecurity`
3. Check whether the participation has `attributes.active = true`
1. If not, discard the participation
4. Save the corresponding person's `first_name`, `last_name`, `nickname` and `email`
8. If the response contains `links.next`, increment the `page` and repeat
9. Return all fetched participations, including each person's `first_name`, `last_name`, `nickname` and `email` as described in the example response above

> Implementation Note: As with the event access control, make sure that the roles that identify a user as a co-leader of an event is configurable per different Hitobito provider, so that new supported instances (i.e. CeviDB) can be easily added in the future.