Lots of tidying

- Removed generic `avr8` target
- Simplified AVR8 target construction
- Introduced register descriptor IDs
- Simplified GDB register mappings
- Simplified target interface contract
- Other bits of tidying

src/DebugServer/Gdb/AvrGdb/CommandPackets/ReadRegister.hpp

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "src/DebugServer/Gdb/CommandPackets/CommandPacket.hpp"
+
+#include "src/DebugServer/Gdb/RegisterDescriptor.hpp"
+
+namespace Bloom::DebugServer::Gdb::AvrGdb::CommandPackets
+{
+    /**
+     * The ReadRegister class implements a structure for the "p" command packet. In response to this packet, the server
+     * is expected to send register values for the requested register.
+     */
+    class ReadRegister: public Gdb::CommandPackets::CommandPacket
+    {
+    public:
+        GdbRegisterId registerId;
+
+        explicit ReadRegister(const RawPacket& rawPacket);
+
+        void handle(
+            DebugSession& debugSession,
+            Services::TargetControllerService& targetControllerService
+        ) override;
+    };
+}