Add plaintext.proto

Update Makefile to handle the import in `plaintext.proto`.
Import path is modified to be relative to the project root.
And we run `protoc` from where `Makefile` locates, i.e. the project
root.

Reference:
- plaintext.proto: 62b2c6c482/sec/insecure/pb/plaintext.proto

libp2p/security/insecure/pb/plaintext_pb2.pyi

@@ -0,0 +1,45 @@
+# @generated by generate_proto_mypy_stubs.py.  Do not edit!
+import sys
+from google.protobuf.descriptor import (
+    Descriptor as google___protobuf___descriptor___Descriptor,
+)
+
+from google.protobuf.message import (
+    Message as google___protobuf___message___Message,
+)
+
+from libp2p.crypto.pb.crypto_pb2 import (
+    PublicKey as libp2p___crypto___pb___crypto_pb2___PublicKey,
+)
+
+from typing import (
+    Optional as typing___Optional,
+)
+
+from typing_extensions import (
+    Literal as typing_extensions___Literal,
+)
+
+
+class Exchange(google___protobuf___message___Message):
+    DESCRIPTOR: google___protobuf___descriptor___Descriptor = ...
+    id = ... # type: bytes
+
+    @property
+    def pubkey(self) -> libp2p___crypto___pb___crypto_pb2___PublicKey: ...
+
+    def __init__(self,
+        *,
+        id : typing___Optional[bytes] = None,
+        pubkey : typing___Optional[libp2p___crypto___pb___crypto_pb2___PublicKey] = None,
+        ) -> None: ...
+    @classmethod
+    def FromString(cls, s: bytes) -> Exchange: ...
+    def MergeFrom(self, other_msg: google___protobuf___message___Message) -> None: ...
+    def CopyFrom(self, other_msg: google___protobuf___message___Message) -> None: ...
+    if sys.version_info >= (3,):
+        def HasField(self, field_name: typing_extensions___Literal[u"id",u"pubkey"]) -> bool: ...
+        def ClearField(self, field_name: typing_extensions___Literal[u"id",u"pubkey"]) -> None: ...
+    else:
+        def HasField(self, field_name: typing_extensions___Literal[u"id",b"id",u"pubkey",b"pubkey"]) -> bool: ...
+        def ClearField(self, field_name: typing_extensions___Literal[u"id",b"id",u"pubkey",b"pubkey"]) -> None: ...