ref: crashpad: pre-generate crash event ID

[jpnurmi]

Pre-generate an event ID for a potential future crash on the client side to be able to associate feedback with the crash event at the time of the crash. Extracted from #1303.

Before, Crashpad events (__sentry-event) were sent without ID. Even if the server generated an ID on demand and sent it as a response, it was too late for the upcoming feedback handler / crash reporter (#1223). It needs an ID before the event is sent because the user might not give consent to send anything at all.

Before

msgpack2json -p -i <run>/__sentry-event

{
    "level": "fatal",
    "platform": "native",
    "release": "sentry-playground@1.2.3",
    "environment": "development",
    "sdk": {
        "name": "sentry.native",
        "version": "0.9.1",
        "packages": [
            {
                "name": "github:getsentry/sentry-native",
                "version": "0.9.1"
            }
        ],
        "integrations": [
            "crashpad"
        ]
    },
    "tags": {
        "backend": "crashpad"
    },
    "extra": {},
    "contexts": {
        "os": {
            "name": "macOS",
            "version": "15.5.0",
            "build": "24F74",
            "kernel_version": "24.5.0"
        },
        "trace": {
            "trace_id": "be7045b229de43014da2b15ceb4dbedf",
            "span_id": "4a800c2ce1b24ebf",
            "sample_rand": 0.3444084273172463
        }
    }
}

After

msgpack2json -p -i <run>/__sentry-event

{
    "event_id": "62e3e0f8-d0d1-450a-1f6e-89faf1fe3862",
    "level": "fatal",
    "platform": "native",
    "release": "sentry-playground@1.2.3",
    "environment": "development",
    "sdk": {
        "name": "sentry.native",
        "version": "0.9.1",
        "packages": [
            {
                "name": "github:getsentry/sentry-native",
                "version": "0.9.1"
            }
        ],
        "integrations": [
            "crashpad"
        ]
    },
    "tags": {
        "backend": "crashpad"
    },
    "extra": {},
    "contexts": {
        "os": {
            "name": "macOS",
            "version": "15.5.0",
            "build": "24F74",
            "kernel_version": "24.5.0"
        },
        "trace": {
            "trace_id": "10b740a911374e5d4d918308abed7cd2",
            "span_id": "2f040e2ccb5b4460",
            "sample_rand": 0.502786153155452
        }
    }
}

[vaind]

LGTM except for the name of the new function

[JoshuaMoelans]

LGTM, minor nit on adding a more clarifying docstring