feat: display state and activity of the retrieval process (#404)

Description

Display state and activity of the retrieval process:

• create a new API endpoint GET /instance/retrieval
• collect activity rows from source and target databases:
  • show activities only for logical mode and refreshing state
  • skip if it's impossible to get activity
  • filter activity by application name - dle_retrieval
  • duration in seconds

Related issue

#404 (closed)

Examples

Response

{
    "mode": "logical",
    "status": "refreshing",
    "lastRefresh": "2022-08-11T08:19:37Z",
    "nextRefresh": null,
    "alerts": {},
    "activity": {
        "source": [
            {
                "user": "john",
                "query": "SET TRANSACTION ISOLATION LEVEL REPEATABLE READ, READ ONLY",
                "duration": 0.083282,
                "waitEventType": "Client",
                "waitEvent": "ClientRead"
            },
            {
                "user": "john",
                "query": "COPY public.pgbench_accounts (aid, bid, abalance, filler) TO stdout;",
                "duration": 6.941369,
                "waitEventType": "Client",
                "waitEvent": "ClientWrite"
            }
        ],
        "target": []
    }
}

Checklist

• MR description has been reviewed
• MR changes are functionally tested
• MR does NOT have text changes OR there are text changes and they have been reviewed
• MR does NOT have API/CLI changes OR there are API/CLI changes and they have been reviewed
• MR does NOT have UI changes OR there are UI changes and they have been reviewed

Closes #404 (closed)