Security: SecureSession

Make security sessions(secio, noise) share the same implementation
`BaseSession` to avoid duplicate implementation of buffered read.

libp2p/security/insecure/io.py

@@ -0,0 +1,19 @@
+from libp2p.io.abc import MsgReadWriteCloser, ReadWriteCloser
+from libp2p.utils import encode_fixedint_prefixed, read_fixedint_prefixed
+
+
+class PlaintextHandshakeReadWriter(MsgReadWriteCloser):
+    conn: ReadWriteCloser
+
+    def __init__(self, conn: ReadWriteCloser) -> None:
+        self.conn = conn
+
+    async def read_msg(self) -> bytes:
+        return await read_fixedint_prefixed(self.conn)
+
+    async def write_msg(self, msg: bytes) -> None:
+        encoded_msg_bytes = encode_fixedint_prefixed(msg)
+        await self.conn.write(encoded_msg_bytes)
+
+    async def close(self) -> None:
+        await self.conn.close()