313539854 Add Pub/Sub as notification backend

[jpassing]

Add Pub/Sub as an optional notification backend. When configured, JIT Access sends
the following three types of notifications to a Pub/Sub topic:

JIT self approval:

{
    type: "ActivationSelfApproved",
    attributes: {
        role: "roles/automl.viewer",
        beneficiary: "alice@example.com",
        start_time: "2023-11-28T22:13:44Z",
        end_time: "2023-11-28T22:23:44Z",
        justification: "...",
        project_id: "project-1"
    }
}

MPA approval request:

{
    "type": "RequestActivation",
    "attributes": {
        "role": "roles/genomics.editor",
        "beneficiary": "alice@example.com",
        "start_time": "2023-11-28T22:19:06Z",
        "end_time": "2023-11-28T22:29:06Z",
        "reviewers": [
            "a-bob@example.com",
            "jitaccess-testuser1@example.com",
            "jitaccess-testuser2@example.com"
        ],
        "justification": "test",
        "action_url": "http://localhost:8080/?activation=JhbGciOi...",
        "request_expiry_time": "2023-11-28T23:19:06Z",
        "base_url": "http://localhost:8080/",
        "project_id": "project-1"
    }
}

MPA approval:

{
    "type": "ActivationApproved",
    "attributes": {
        "role": "roles/genomics.editor",
        "beneficiary": "alice@example.com",
        "start_time": "2023-11-28T22:19:06Z",
        "end_time": "2023-11-28T22:29:06Z",
        "reviewers": [
            "a-bob@example.com",
            "jitaccess-testuser1@example.com",
            "jitaccess-testuser2@example.com"
        ],
        "justification": "test",
        "base_url": "http://localhost:8080/",
        "approver": "a-bob@example.com",
        "project_id": "project-1"
    }
}

This PR is derived from #154.