Add an insecure_disable_tls_host_verification command line option

When set, this disables the host verification of all HTTP requests. Available
for both the fetch and serve mode.

Also introduced an App.Config, for future command line options which need to
be passed more deeply into the code.

src/main_unit_tests.zig

@@ -213,7 +213,7 @@ fn serveHTTPS(address: std.net.Address) !void {
 
 fn serveCDP(address: std.net.Address) !void {
     const App = @import("app.zig").App;
-    var app = try App.init(gpa.allocator(), .serve);
+    var app = try App.init(gpa.allocator(), .{.run_mode = .serve});
     defer app.deinit();
 
     const server = @import("server.zig");