feat: add readonly delegation

Signed-off-by: Grigory Vodyanov <scratchx@gmx.com>

Author: GVodyanov

src/index.js

@@ -638,6 +638,38 @@ export default class DavClient {
 			.filter(Boolean)
 	}
 
+	/**
+	 * Returns the principal URLs and permission level of users who have granted
+	 * the given principal proxy access (both read and write).
+	 *
+	 * Inspects the group-membership property for groups ending in
+	 * /calendar-proxy-write or /calendar-proxy-read and returns objects with
+	 * the owner's principal URL and the permission granted.
+	 *
+	 * @param {string} principalUrl Absolute URL of the principal
+	 * @return {Promise<Array<{principalUrl: string, permission: 'write'|'read'}>>}
+	 */
+	async getDelegatorsWithPermission(principalUrl) {
+		const groups = await this.getGroupMembership(principalUrl)
+		const result = []
+
+		for (const groupUrl of groups) {
+			if (groupUrl.includes('calendar-proxy-write')) {
+				const ownerUrl = groupUrl.replace(/\/calendar-proxy-write\/?$/, '')
+				if (ownerUrl) {
+					result.push({ principalUrl: ownerUrl, permission: 'write' })
+				}
+			} else if (groupUrl.includes('calendar-proxy-read')) {
+				const ownerUrl = groupUrl.replace(/\/calendar-proxy-read\/?$/, '')
+				if (ownerUrl) {
+					result.push({ principalUrl: ownerUrl, permission: 'read' })
+				}
+			}
+		}
+
+		return result
+	}
+
 	/**
 	 * discovers all calendar-homes in this account, all principal collections
 	 * and advertised features