Google Calendar Integration: Invite volunteer to hardcoded calendar on register

[MehekB]

Description

Began the integration of Google Calendar into the event workflow.

• Implemented a proof of concept that sends a Google Calendar invite to a volunteer when they RSVP to an event. The invite is currently generated from a hardcoded calendar event to validate that the flow works end-to-end.
• Added documentation outlining how to create Google Calendar events using the GCal API, which will be used once the event creation feature is finalized.
• Added documentation on setting up Google Cloud and enabling the Google Calendar API.

Documentation:

• GCal Event Creation Guide
• Google Cloud / GCal Setup

Issues

Closes #104