Add configsource component (#2720)

* Add configsource component

Create the new component with  minimal code addition.

* Add ApplyConfigSources to a configuration

* Add ApplyConfigSources to a configuration

* Add ConfigSourceSettings

* Add local expandEnvConfig to configsource package

Avoids exposing it from config package.

* Relocate configsource to config/interal

* Move ConfigSource from component package to under configsource

* Fixes after files moved

* Make configmodels.ConfigSource internal

* Add comments and some renames

* Fix deepestConfigSourcesFirst

Fix whitespace

* Fix whitespace

* Do not allow nested config source invocations

* Undo accidental change to go.sum

* PR Feedback

* Reducing scope: keeping only config source interfaces

* Remove boiler plate code that can be added later

* Reduce the interface to a minimum

* Return error from Session.Close

config/internal/configsource/component.go

@@ -0,0 +1,52 @@
+// 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 configsource
+
+import (
+	"context"
+)
+
+// ConfigSource is the interface to be implemented by objects used by the collector
+// to retrieve external configuration information.
+type ConfigSource interface {
+	// NewSession must create a Session object that will be used to inject data into
+	// a configuration.
+	//
+	// The Session object should use its creation according to their ConfigSource needs:
+	// lock resources, suspend background tasks, etc. An implementation, for instance,
+	// can use the creation of the Session object to prevent torn configurations,
+	// by acquiring a lock (or some other mechanism) that prevents concurrent changes to the
+	// configuration during the middle of a session.
+	//
+	// The code managing the returned Session object must guarantee that the object is not used
+	// concurrently.
+	NewSession(ctx context.Context) (Session, error)
+}
+
+// Session is the interface used to inject configuration data from a ConfigSource. A Session
+// object is created from a ConfigSource. The code using Session objects must guarantee that
+// methods of a single instance are not called concurrently.
+type Session interface {
+	// Apply goes to the configuration source, and according to the specified selector and
+	// parameters retrieves a configuration value that is injected into the configuration.
+	//
+	// The selector is a string that is required on all invocations, the params are optional.
+	Apply(ctx context.Context, selector string, params interface{}) (interface{}, error)
+
+	// Close signals that the object won't be used anymore to inject data into a configuration.
+	// Each Session object should use this call according to their needs: release resources,
+	// close communication channels, etc.
+	Close(ctx context.Context) error
+}