internal/auth: Add native NetAuth authentication integration.

NetAuth is an easy to deploy and manage organizational source of truth
for identity and group management.  This commit adds support for maddy
to ask NetAuth about authentication using the native protocol.

Author: the-maldridge

go.mod

@@ -62,6 +62,7 @@ require (
 	github.com/digitalocean/godo v1.81.0 // indirect
 	github.com/dustin/go-humanize v1.0.0 // indirect
 	github.com/emersion/go-textwrapper v0.0.0-20200911093747-65d896831594 // indirect
+	github.com/fsnotify/fsnotify v1.4.9 // indirect
 	github.com/go-asn1-ber/asn1-ber v1.5.4 // indirect
 	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
@@ -70,20 +71,27 @@ require (
 	github.com/googleapis/enterprise-certificate-proxy v0.1.0 // indirect
 	github.com/googleapis/gax-go/v2 v2.4.0 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
+	github.com/hashicorp/go-hclog v0.9.2 // indirect
 	github.com/hashicorp/go-retryablehttp v0.7.1 // indirect
+	github.com/hashicorp/hcl v1.0.0 // indirect
 	github.com/jmespath/go-jmespath v0.4.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/compress v1.15.6 // indirect
 	github.com/klauspost/cpuid/v2 v2.0.14 // indirect
+	github.com/magiconair/properties v1.8.5 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
 	github.com/mholt/acmez v1.0.2 // indirect
 	github.com/minio/md5-simd v1.1.2 // indirect
 	github.com/minio/sha256-simd v1.0.0 // indirect
 	github.com/mitchellh/go-homedir v1.1.0 // indirect
+	github.com/mitchellh/mapstructure v1.4.1 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/netauth/netauth v0.6.2-0.20220831214440-1df568cd25d6 // indirect
+	github.com/netauth/protocol v0.0.0-20210918062754-7fee492ffcbd // indirect
+	github.com/pelletier/go-toml v1.9.3 // indirect
 	github.com/pierrec/lz4 v2.6.1+incompatible // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
@@ -94,6 +102,12 @@ require (
 	github.com/ryszard/goskiplist v0.0.0-20150312221310-2dfbae5fcf46 // indirect
 	github.com/shabbyrobe/gocovmerge v0.0.0-20180507124511-f6ea450bfb63 // indirect
 	github.com/sirupsen/logrus v1.8.1 // indirect
+	github.com/spf13/afero v1.6.0 // indirect
+	github.com/spf13/cast v1.3.1 // indirect
+	github.com/spf13/jwalterweatherman v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+	github.com/spf13/viper v1.8.1 // indirect
+	github.com/subosito/gotenv v1.2.0 // indirect
 	github.com/vultr/govultr/v2 v2.17.2 // indirect
 	github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673 // indirect
 	go.opencensus.io v0.23.0 // indirect
@@ -109,6 +123,7 @@ require (
 	google.golang.org/grpc v1.47.0 // indirect
 	google.golang.org/protobuf v1.28.0 // indirect
 	gopkg.in/ini.v1 v1.66.6 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gotest.tools v2.2.0+incompatible // indirect
 )