provider: Add -debug flag to main binary (hashicorp#16565)

Reference: https://github.com/hashicorp/terraform-provider-scaffolding/blob/3c87a86ff2eeb6b0835e26b16f2bf8bc293622fd/main.go#L22-L38

To match recent updates to the scaffold provider, this allows easier usage of Go debugging tooling, such as delve, via a provider reattachment mode. This is primarily for provider developers and does not affect practitioner usage.

Example usage:

```console
$ go run . -debug
{"@Level":"debug","@message":"plugin address","@timestamp":"2020-12-02T21:23:22.095285-05:00","address":"/var/folders/w8/05f3x02n27x72g0mc2jy6_180000gp/T/plugin326120673","network":"unix"}
Provider started, to attach Terraform set the TF_REATTACH_PROVIDERS env var:

	TF_REATTACH_PROVIDERS='{"registry.terraform.io/hashicorp/aws":{"Protocol":"grpc","Pid":45042,"Test":true,"Addr":{"Network":"unix","String":"/var/folders/w8/05f3x02n27x72g0mc2jy6_180000gp/T/plugin326120673"}}}'
```

main.go

@@ -1,11 +1,31 @@
 package main
 
 import (
+	"context"
+	"flag"
+	"log"
+
 	"github.com/hashicorp/terraform-plugin-sdk/v2/plugin"
 	"github.com/terraform-providers/terraform-provider-aws/aws"
 )
 
 func main() {
-	plugin.Serve(&plugin.ServeOpts{
-		ProviderFunc: aws.Provider})
+	var debugMode bool
+
+	flag.BoolVar(&debugMode, "debug", false, "set to true to run the provider with support for debuggers like delve")
+	flag.Parse()
+
+	opts := &plugin.ServeOpts{ProviderFunc: aws.Provider}
+
+	if debugMode {
+		err := plugin.Debug(context.Background(), "registry.terraform.io/hashicorp/aws", opts)
+
+		if err != nil {
+			log.Fatal(err.Error())
+		}
+
+		return
+	}
+
+	plugin.Serve(opts)
 }