Services: Rename ProtocolServer to RequestServer

The current ProtocolServer was really only used for requests, and with
the recent introduction of the WebSocket service, long-lasting
connections with another server are not part of it. To better reflect
this, this commit renames it to RequestServer.

This commit also changes the existing 'protocol' portal to 'request',
the existing 'protocol' user and group to 'request', and most mentions
of the 'download' aspect of the request to 'request' when relevant, to
make everything consistent across the system.

Note that LibProtocol still exists as-is, but the more generic Client
class and the more specific Download class have both been renamed to a
more accurate RequestClient and Request to match the new names.

This commit only change names, not behaviors.

Userland/Services/RequestServer/RequestServer.ipc

@@ -0,0 +1,12 @@
+endpoint RequestServer
+{
+    // Basic protocol
+    Greet() => ()
+
+    // Test if a specific protocol is supported, e.g "http"
+    IsSupportedProtocol(String protocol) => (bool supported)
+
+    StartRequest(String method, URL url, IPC::Dictionary request_headers, ByteBuffer request_body) => (i32 request_id, Optional<IPC::File> response_fd)
+    StopRequest(i32 request_id) => (bool success)
+    SetCertificate(i32 request_id, String certificate, String key) => (bool success)
+}