Add worker tagging (#862)

This allows workers to have tags assigned to them, which can then be filtered to control which targets they are allowed to serve.

go.mod

@@ -23,6 +23,7 @@ require (
 	github.com/hashicorp/boundary/sdk v0.0.1
 	github.com/hashicorp/dbassert v0.0.0-20200930125617-6218396928df
 	github.com/hashicorp/errwrap v1.1.0
+	github.com/hashicorp/go-bexpr v0.1.4
 	github.com/hashicorp/go-cleanhttp v0.5.1
 	github.com/hashicorp/go-hclog v0.15.0
 	github.com/hashicorp/go-kms-wrapping v0.5.16
@@ -43,6 +44,7 @@ require (
 	github.com/mitchellh/cli v1.1.2
 	github.com/mitchellh/go-wordwrap v1.0.1
 	github.com/mitchellh/gox v1.0.1
+	github.com/mitchellh/mapstructure v1.3.3
 	github.com/mr-tron/base58 v1.2.0
 	github.com/oligot/go-mod-upgrade v0.3.0
 	github.com/ory/dockertest/v3 v3.6.0