feature request: add option to allow calendar creation for guests when auth is enabled

[panteLx]

Checklist

• I checked that this feature has not been requested before
• I checked that this feature is not in the "wontfix / not planned" list
• This feature will benefit the majority of users

Problem Description / Use Case

currently its not possible without an account to create new calendars.

Proposed Solution

https://www.better-auth.com/docs/plugins/anonymous