server: support OAI /v1/audio/transcriptions API (#21863)

* server: support OAI /v1/audio/transcriptions API

* address autoreview comments

* correct default response_format value

tools/server/server-http.h

@@ -5,6 +5,8 @@
 #include <map>
 #include <string>
 #include <thread>
+#include <vector>
+#include <cstdint>
 
 struct common_params;
 
@@ -32,6 +34,7 @@ struct server_http_res {
 // unique pointer, used by set_chunked_content_provider
 // httplib requires the stream provider to be stored in heap
 using server_http_res_ptr = std::unique_ptr<server_http_res>;
+using raw_buffer = std::vector<uint8_t>;
 
 struct server_http_req {
     std::map<std::string, std::string> params; // path_params + query_params
@@ -39,6 +42,7 @@ struct server_http_req {
     std::string path;
     std::string query_string; // query parameters string (e.g. "action=save")
     std::string body;
+    std::map<std::string, raw_buffer> files; // used for file uploads (form data)
     const std::function<bool()> & should_stop;
 
     std::string get_param(const std::string & key, const std::string & def = "") const {