CalDAV support

[emersion]

Ref #207

[chaudl113]

/claim #340

Submitted fix in PR #342. The implementation adds a CalDAV server that bridges ProtonMail's calendar API to the CalDAV protocol.

The implementation follows the same pattern as the existing CardDAV backend:

• New caldav/ package implementing the go-webdav CalDAV Backend interface
• Fetches calendars and events from ProtonMail's calendar API
• Converts events to iCal format
• New CLI command and flags for CalDAV server
• Integrated into the serve command

[cjroth]

Working in #343