Add a first pass at a gcal connector

[seanstory]

Part of https://github.com/elastic/search-team/issues/11334

Sample synced cal event:

{
        "_index": "connector-gcal-2",
        "_id": "0e2pmh25c8heckt9h5gi0jemj0",
        "_score": 5.428957,
        "_source": {
          "end_date": null,
          "start_datetime": "2025-07-24T16:00:00-04:00",
          "end_datetime": "2025-07-24T17:00:00-04:00",
          "created_at": "2025-07-23T19:38:11+00:00",
          "description": "This is a very important, very real meeting for international business people on international business trips.",
          "type": "event",
          "event_type": "default",
          "updated_at": "2025-07-23T19:38:12.129000+00:00",
          "html_link": "https://www.google.com/calendar/event?eid=MGUycG1oMjVjOGhlY2t0OWg1Z2kwamVtajAgYWRtaW5AZXh0cmFjdC10cmFuc2Zvcm0uY29t",
          "id": "0e2pmh25c8heckt9h5gi0jemj0",
          "_timestamp": "2025-07-23T19:39:26.302102+00:00",
          "start_date": null,
          "calendar": {
            "name": "[email protected]",
            "id": "[email protected]",
            "type": "calendar"
          },
          "summary": "Super important meeting",
          "creator": {
            "self": true,
            "email": "[email protected]"
          },
          "calendar_id": "[email protected]",
          "visibility": null,
          "attendees": [
            {
              "organizer": true,
              "self": true,
              "responseStatus": "accepted",
              "email": "[email protected]"
            },
            {
              "responseStatus": "needsAction",
              "email": "[email protected]"
            }
          ],
          "recurring_event_id": null,
          "recurrence": null,
          "event_id": "0e2pmh25c8heckt9h5gi0jemj0",
          "color_id": null,
          "organizer": {
            "self": true,
            "email": "[email protected]"
          },
          "transparency": null,
          "conference_data": {
            "entryPoints": [
              {
                "entryPointType": "video",
                "label": "meet.google.com/qam-bptk-odr",
                "uri": "https://meet.google.com/qam-bptk-odr"
              },
              {
                "pin": "7369427064877",
                "entryPointType": "more",
                "uri": "https://tel.meet/qam-bptk-odr?pin=7369427064877"
              },
              {
                "regionCode": "US",
                "pin": "839137256",
                "entryPointType": "phone",
                "label": "+1 409-420-7434",
                "uri": "tel:+1-409-420-7434"
              }
            ],
            "conferenceId": "qam-bptk-odr",
            "conferenceSolution": {
              "name": "Google Meet",
              "key": {
                "type": "hangoutsMeet"
              },
              "iconUri": "https://fonts.gstatic.com/s/i/productlogos/meet_2020q4/v6/web-512dp/logo_meet_2020q4_color_2x_web_512dp.png"
            }
          },
          "location": null,
          "status": "confirmed"
        }
      }

Checklists

Pre-Review Checklist

• this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• this PR has a meaningful title
• this PR links to all relevant github issues that it fixes or partially addresses
• if there is no GH issue, please create it. Each PR should have a link to an issue
• this PR has a thorough description
• Covered the changes with automated tests
• Tested the changes locally
• Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• Considered corresponding documentation changes

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