Implement link previews for text messages using Apple's LinkPreview framework (#4520)

ElementX/Sources/Services/LinkMetadata/LinkMetadataProviderProtocol.swift

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+//
+// Copyright 2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Foundation
+import LinkPresentation
+
+struct LinkMetadataProviderItem {
+    let url: URL
+    let metadata: LPLinkMetadata?
+}
+
+protocol LinkMetadataProviderProtocol {
+    var metadataItems: [URL: LinkMetadataProviderItem] { get }
+    
+    func fetchMetadataFor(url: URL) async -> Result<LinkMetadataProviderItem, Error>
+}