Contrib: Add an exemplar_util artifact to help record special exemplars. (#1297)

* Stats: Add attachment key for TraceId and SpanId.

* Move the keys to an independent artifact and add methods for encoding.

* Add this change to CHANGELOG.

* Update description and tests.

* Have one method that takes SpanContext instead of two.

* Rename method to putSpanContextAttachments.
diff --git a/settings.gradle b/settings.gradle
index 6713159..036df52 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -15,6 +15,7 @@
 include ":opencensus-exporter-stats-prometheus"
 include ":opencensus-contrib-agent"
 include ":opencensus-contrib-appengine-standard-util"
+include ":opencensus-contrib-exemplar-util"
 include ":opencensus-contrib-grpc-metrics"
 include ":opencensus-contrib-grpc-util"
 include ":opencensus-contrib-http-util"
@@ -30,6 +31,7 @@
 project(':opencensus-contrib-agent').projectDir = "$rootDir/contrib/agent" as File
 project(':opencensus-contrib-appengine-standard-util').projectDir =
         "$rootDir/contrib/appengine_standard_util" as File
+project(':opencensus-contrib-exemplar-util').projectDir = "$rootDir/contrib/exemplar_util" as File
 project(':opencensus-contrib-grpc-metrics').projectDir = "$rootDir/contrib/grpc_metrics" as File
 project(':opencensus-contrib-grpc-util').projectDir = "$rootDir/contrib/grpc_util" as File
 project(':opencensus-contrib-http-util').projectDir = "$rootDir/contrib/http_util" as File