Add Version func to otel/sdk (#3949)

* Add Version func to otel/sdk

* Update sdk/resource to use sdk version

* Remove unused UserAgent from sdk/internal

* Add changes to changelog

* Update CHANGELOG.md

Co-authored-by: Robert Pająk <pellared@hotmail.com>

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

sdk/version.go

@@ -0,0 +1,20 @@
+// Copyright The 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.
+
+package sdk // import "go.opentelemetry.io/otel/sdk"
+
+// Version is the current release version of the OpenTelemetry SDK in use.
+func Version() string {
+	return "1.15.0-rc.2"
+}