Add FileCDN class enable generating signedURL's in File Attachments

[bpollman]

🔗 Issue Link

Jira or Github issue link, if applicable.

🎯 Goal

To use our own CDN, we need the equivalent of ImageCDN to intercept a given URL and provide a signed URL when dealing with File attachments.

🛠 Implementation

Example of how we are using it to make an async call for a signed URL.

public final class CustomFileCDN: FileCDN {

    static var customCDNURL = "mycustomCDN.com"

    let attachmentService: ChatAttachmentService

    init(attachmentService: ChatAttachmentService) {
        self.attachmentService = attachmentService
    }

    public func adjustedURL(for url: URL, completion: @escaping ((Result<URL, any Error>) -> Void)) {
        Task {
            do {
                let signedUrl = try await signedUrl(for: url)
                completion(.success(signedUrl))

            } catch {
                completion(.failure(error))
            }
        }
    }

    func signedUrl(for url: URL) async throws -> URL {
        guard
            let components = URLComponents(url: url, resolvingAgainstBaseURL: true),
            let host = components.host,
            host.contains(Self.customCDNURL)
        else {
            return url
        }
        return try await self.attachmentService.signedDownloadUrl(url)
    }
}

🧪 Testing

Describe the steps how this change can be tested (or why it can't be tested).

🎨 Changes

Add relevant screenshots or videos showcasing the changes.

☑️ Checklist

• [x] I have signed the Stream CLA (required)
• [ ] Changelog is updated with client-facing changes
• [ ] New code is covered by unit tests
• [ ] Affected documentation updated (docusaurus, tutorial, CMS (task created)