perf(client): enable HTTP/2 and brotli support in internal HTTP client

- Prefer HTTP/2 by setting RequestOptions::VERSION => "2.0" so clients
  that respect PSR-7 request version will prefer HTTP/2.
- Add a curl hint (CURLOPT_HTTP_VERSION) to prefer HTTP/2 via ALPN
  (CURL_HTTP_VERSION_2TLS or CURL_HTTP_VERSION_2_0 fallback) while allowing
  automatic fallback to HTTP/1.1.
- Advertise Brotli ("br") in Accept-Encoding when the php-brotli extension
  is available (detected via function_exists('brotli_uncompress')), otherwise
  fall back to gzip.

Notes:
- The PSR-7 request version is used as a hint for HTTP client libraries;
  setting the version to "2.0" signals a preference for HTTP/2 at the request
  abstraction level.
- The curl option is defensive: it prefers HTTP/2 where libcurl supports it
  (via ALPN), but will not break on older libcurl/builds (uses defined()).

Compatibility:
- Fully backwards compatible: if the php-brotli extension is not present,
  no Brotli usage will occur and behaviour remains equivalent to previous
  (gzip).

Signed-off-by: ernolf <raphael.gradenwitz@googlemail.com>

Author: ernolf

lib/private/Http/Client/Client.php

@@ -54,7 +54,14 @@ private function buildRequestOptions(array $options): array {
 		$defaults = [
 			RequestOptions::VERIFY => $this->getCertBundle(),
 			RequestOptions::TIMEOUT => IClient::DEFAULT_REQUEST_TIMEOUT,
+			// Prefer HTTP/2 globally (PSR-7 request version)
+			RequestOptions::VERSION => '2.0',
 		];
+		// cURL hint: Prefer HTTP/2 (with ALPN); automatically falls back to 1.1.
+		$defaults['curl'][\CURLOPT_HTTP_VERSION]
+			= \defined('CURL_HTTP_VERSION_2TLS') ? \CURL_HTTP_VERSION_2TLS
+			: (\defined('CURL_HTTP_VERSION_2_0') ? \CURL_HTTP_VERSION_2_0
+			: \CURL_HTTP_VERSION_NONE);
 
 		$options['nextcloud']['allow_local_address'] = $this->isLocalAddressAllowed($options);
 		if ($options['nextcloud']['allow_local_address'] === false) {
@@ -84,8 +91,15 @@ private function buildRequestOptions(array $options): array {
 			$options[RequestOptions::HEADERS]['User-Agent'] = 'Nextcloud Server Crawler';
 		}
 
-		if (!isset($options[RequestOptions::HEADERS]['Accept-Encoding'])) {
-			$options[RequestOptions::HEADERS]['Accept-Encoding'] = 'gzip';
+		// Ensure headers array exists and set Accept-Encoding only if not present
+		$headers = $options[RequestOptions::HEADERS] ?? [];
+		if (!isset($headers['Accept-Encoding'])) {
+			$acceptEnc = 'gzip';
+			if (function_exists('brotli_uncompress')) {
+				$acceptEnc = 'br, ' . $acceptEnc;
+			}
+			$options[RequestOptions::HEADERS] = $headers; // ensure headers are present
+			$options[RequestOptions::HEADERS]['Accept-Encoding'] = $acceptEnc;
 		}
 
 		// Fallback for save_to