[CON-3243] feat: Add google calendar subscriber connector

[Jkarage]

Implements Subscribe, UpdateSubscription, DeleteSubscription, and RunScheduledMaintenance for the Google Calendar connector using the Calendar Watch API.

Objects supported:

• events — POST /calendars/primary/events/watch
• calendarList — POST /users/me/calendarList/watch
• settings — POST /users/me/settings/watch
• acl — POST /calendars/primary/acl/watch

Live Tests:

Google Calendar push notifications are delivered as HTTP POST requests to the address set in WatchRequest.Address. The body is always empty. All
information arrives in headers:

Header: X-Goog-Resource-State
Values: sync
Meaning: Initial handshake on channel creation — ignore it
Values: exists
Meaning: A resource was created or updated
Values: not_exists
Meaning: A resource was deleted

Header: X-Goog-Channel-ID
Values: e.g. -events
Meaning: The channel ID set during subscribe

Header: X-Goog-Resource-ID
Values: opaque string
Meaning: Stable ID of the watched resource (needed to stop the channel)

Header: X-Goog-Resource-URI
Values: URL
Meaning: The API endpoint of the changed resource

Header: X-Goog-Channel-Token
Values: string
Meaning: The token set in WatchRequest.Token, echoed back verbatim — use this to verify the notification came from Google

Header: X-Goog-Channel-Expiration
Values: RFC 1123 date
Meaning: When the channel expires

[linear]

CON-3243