CalDAV Digest Authentication Support Missing

[DmitriSchrama]

Description:
Easy!Appointments CalDAV sync fails with 401 Unauthorized when connecting to CalDAV servers that require Digest authentication (e.g., SabreDAV-based servers like zxcs.nl).

Steps to Reproduce:

1. Set up a CalDAV server with Digest authentication (SabreDAV default)
2. Configure Easy!Appointments CalDAV sync with server credentials
3. Attempt to connect or sync
4. Observe 401 Unauthorized error with message: "No 'Authorization: Digest' header found"

Expected Behavior:
CalDAV sync should work with both Basic and Digest authentication methods.

Actual Behavior:
Only Basic authentication is supported, causing connection failures with Digest-only servers.

Root Cause:
In application/libraries/Caldav_sync.php, the get_http_client() method uses Basic authentication only:

'auth' => [$caldav_username, $caldav_password],

Proposed Solution:
Add Digest authentication support:

// Option 1: Always use Digest (may break Basic-only servers)
'auth' => [$caldav_username, $caldav_password, 'digest'],

// Option 2: Configurable authentication type
$authType = $provider['settings']['caldav_auth_type'] ?? 'basic';
'auth' => [$caldav_username, $caldav_password, $authType],

// Option 3: Auto-detect based on server response

Affected Version:
Easy!Appointments v1.5.0+

Workaround:
Manually edit Caldav_sync.php to add 'digest' as the third parameter in the auth array.

Additional Context:

• Tested with zxcs.nl CalDAV server (SabreDAV 4.4.0)
• Error: Sabre\DAV\Exception\NotAuthenticated: No 'Authorization: Digest' header found
• Fix verified working with Digest authentication