server: add built-in tools backend support (#20898)

* wip: server_tools

* refactor

* displayName -> display_name

* snake_case everywhere

* rm redundant field

* change arg to --tools all

* add readme mention

* llama-gen-docs

tools/server/server-tools.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include "server-common.h"
+#include "server-http.h"
+
+struct server_tool {
+    std::string name;
+    std::string display_name;
+    bool permission_write = false;
+
+    virtual ~server_tool() = default;
+    virtual json get_definition() = 0;
+    virtual json invoke(json params) = 0;
+
+    json to_json();
+};
+
+struct server_tools {
+    std::vector<std::unique_ptr<server_tool>> tools;
+
+    void setup(const std::vector<std::string> & enabled_tools);
+    json invoke(const std::string & name, const json & params);
+
+    server_http_context::handler_t handle_get;
+    server_http_context::handler_t handle_post;
+};