Add .proto files and generate Go Protobufs

CONTRIBUTING.md

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+# Contributing to opamp-go library
+
+## Generate Protobuf Go Files
+
+Make sure you have Docker installed.
+
+Run `make gen-proto`. This should compile `internal/proto/*.proto` files to 
+`internal/protobufs/*.pb.go` files.

go.mod

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+module github.com/open-telemetry/opamp-go
+
+go 1.17
+
+require google.golang.org/protobuf v1.27.1

go.sum

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+github.com/golang/protobuf v1.5.0/go.mod h1:FsONVRAS9T7sI+LIUmWTfcYkHO4aIWwzhcaSAoJOfIk=
+github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
+github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+google.golang.org/protobuf v1.26.0-rc.1/go.mod h1:jlhhOSvTdKEhbULTjvd4ARK9grFBp09yW+WbY/TyQbw=
+google.golang.org/protobuf v1.27.1 h1:SnqbnDw1V7RiZcXPx5MEeqPv2s79L9i7BJUlG/+RurQ=
+google.golang.org/protobuf v1.27.1/go.mod h1:9q0QmTI4eRPtz6boOQmLYwt+qCgq0jsYwAQnmE0givc=

internal/proto/anyvalue.proto

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+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file is copied and modified from https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/common/v1/common.proto
+// Modifications:
+//  - Removal of unneeded InstrumentationLibrary and StringKeyValue messages.
+//  - Change of go_package to reference a package in this repo.
+//  - Removal of gogoproto usage.
+
+syntax = "proto3";
+
+package opamp.proto;
+
+option go_package = "github.com/open-telemetry/opamp-go/internal/protobufs";
+
+// AnyValue is used to represent any type of attribute value. AnyValue may contain a
+// primitive value such as a string or integer or it may contain an arbitrary nested
+// object containing arrays, key-value lists and primitives.
+message AnyValue {
+  // The value is one of the listed fields. It is valid for all values to be unspecified
+  // in which case this AnyValue is considered to be "null".
+  oneof value {
+    string string_value = 1;
+    bool bool_value = 2;
+    int64 int_value = 3;
+    double double_value = 4;
+    ArrayValue array_value = 5;
+    KeyValueList kvlist_value = 6;
+    bytes bytes_value = 7;
+  }
+}
+
+// ArrayValue is a list of AnyValue messages. We need ArrayValue as a message
+// since oneof in AnyValue does not allow repeated fields.
+message ArrayValue {
+  // Array of values. The array may be empty (contain 0 elements).
+  repeated AnyValue values = 1;
+}
+
+// KeyValueList is a list of KeyValue messages. We need KeyValueList as a message
+// since `oneof` in AnyValue does not allow repeated fields. Everywhere else where we need
+// a list of KeyValue messages (e.g. in Span) we use `repeated KeyValue` directly to
+// avoid unnecessary extra wrapping (which slows down the protocol). The 2 approaches
+// are semantically equivalent.
+message KeyValueList {
+  // A collection of key/value pairs of key-value pairs. The list may be empty (may
+  // contain 0 elements).
+  repeated KeyValue values = 1;
+}
+
+// KeyValue is a key-value pair that is used to store Span attributes, Link
+// attributes, etc.
+message KeyValue {
+  string key = 1;
+  AnyValue value = 2;
+}