Introduce Endpoint#messages_received_per_type

[Al2Klimov]

Counts incoming messages per type and endpoint. Queryable via API:

Test

> GET /v1/objects/endpoints?pretty=1 HTTP/1.1
> Host: 127.0.0.1:5665
> Authorization: Basic cm9vdDoxMjM0NTY=
> User-Agent: curl/8.7.1
> Accept: application/json
> Content-Length: 40
> Content-Type: application/x-www-form-urlencoded
>
* upload completely sent off: 40 bytes
< HTTP/1.1 200 OK
< Server: Icinga/v2.14.0-488-gbf0a2d4c3
< Content-Type: application/json
< Content-Length: 317
<
{
    "results": [
        {
            "attrs": {
                "messages_received_per_type": {
                    "icinga::Hello": 1
                }
            },
            "joins": {},
            "meta": {},
            "name": "ws-aklimov7777777.local",
            "type": "Endpoint"
        }
    ]
}

👍

[Al2Klimov]

Idea

1. std::unordered_map#reserve() twice as many as needed
2. std::hash collisions will be even less likely at lookup
3. Lookup gets even faster 😎