New schemas tfstates3 and tfstategs (#49)

Adds new schemes that can allow uses of direct path to Terraform state in the bucket: one for GCS another for S3.

The change requires #47 to be applied first as it supports GCS buckets implemented by tfstate-lookup v0.2.0

Resolves #45

README.md

@@ -335,6 +335,48 @@ Remote backends like S3 or GCS are also supported. When a remote backend is used
 
 Just specify the path to that file, so that `vals` is able to transparently make the remote state contents available for you.
 
+### Terraform in GCS bucket (tfstategs)
+
+- `ref+tfstategs://bucket/path/to/some.tfstate/RESOURCE_NAME`
+
+Examples:
+
+- `ref+tfstategs://bucket/path/to/some.tfstate/google_compute_disk.instance.id`
+
+It allows to use Terraform state stored in GCS bucket with the direct URL to it. You can try to read the state with command: 
+
+```
+$ tfstate-lookup -s gs://bucket-with-terraform-state/terraform.tfstate google_compute_disk.instance.source_image_id
+5449927740744213880
+```
+
+which is equivalent to the following input for `vals`:
+
+```
+$ echo 'foo: ref+tfstategs://bucket-with-terraform-state/terraform.tfstate/google_compute_disk.instance.source_image_id' | vals eval -f -
+```
+
+### Terraform in S3 bucket (tfstates3)
+
+- `ref+tfstates3://bucket/path/to/some.tfstate/RESOURCE_NAME`
+
+Examples:
+
+- `ref+tfstates3://bucket/path/to/some.tfstate/aws_vpc.main.id`
+
+It allows to use Terraform state stored in AWS S3 bucket with the direct URL to it. You can try to read the state with command: 
+
+```
+$ tfstate-lookup -s s3://bucket-with-terraform-state/terraform.tfstate module.vpc.aws_vpc.this[0].arn
+arn:aws:ec2:us-east-2:ACCOUNT_ID:vpc/vpc-0cb48a12e4df7ad4c
+```
+
+which is equivalent to the following input for `vals`:
+
+```
+$ echo 'foo: ref+tfstates3://bucket-with-terraform-state/terraform.tfstate/module.vpc.aws_vpc.this[0].arn' | vals eval -f -
+```
+
 ### SOPS
 
 - The whole content of a SOPS-encrypted file: `ref+sops://base64_data_or_path_to_file?key_type=[filepath|base64]&format=[binary|dotenv|yaml]`

pkg/providers/tfstate/tfstate.go

@@ -11,10 +11,12 @@ import (
 )
 
 type provider struct {
+	backend string
 }
 
-func New(cfg api.StaticConfig) *provider {
+func New(cfg api.StaticConfig, backend string) *provider {
 	p := &provider{}
+	p.backend = backend
 	return p
 }
 
@@ -27,9 +29,9 @@ func (p *provider) GetString(key string) (string, error) {
 	f := strings.Join(splits[:pos], string(os.PathSeparator))
 	k := strings.Join(splits[pos:], string(os.PathSeparator))
 
-	state, err := tfstate.ReadFile(f)
+	state, err := p.ReadTFState(f, k)
 	if err != nil {
-		return "", fmt.Errorf("reading tfstate for %s: %w", key, err)
+		return "", err
 	}
 
 	// key is something like "aws_vpc.main.id" (RESOURCE_TYPE.RESOURCE_NAME.FIELD)
@@ -42,6 +44,25 @@ func (p *provider) GetString(key string) (string, error) {
 	return attrs.String(), nil
 }
 
+// Read state either from file or from backend
+func (p *provider) ReadTFState(f, k string) (*tfstate.TFState, error) {
+	switch p.backend {
+	case "":
+		state, err := tfstate.ReadFile(f)
+		if err != nil {
+			return nil, fmt.Errorf("reading tfstate for %s: %w", k, err)
+		}
+		return state, nil
+	default:
+		url := p.backend + "://" + f
+		state, err := tfstate.ReadURL(url)
+		if err != nil {
+			return nil, fmt.Errorf("reading tfstate for %s: %w", k, err)
+		}
+		return state, nil
+	}
+}
+
 func (p *provider) GetStringMap(key string) (map[string]interface{}, error) {
 	return nil, fmt.Errorf("path fragment is not supported for tfstate provider")
 }

pkg/stringprovider/stringprovider.go

@@ -31,7 +31,11 @@ func New(provider api.StaticConfig) (api.LazyLoadedStringProvider, error) {
 	case "gcpsecrets":
 		return gcpsecrets.New(provider), nil
 	case "tfstate":
-		return tfstate.New(provider), nil
+		return tfstate.New(provider, ""), nil
+	case "tfstategs":
+		return tfstate.New(provider, "gs"), nil
+	case "tfstates3":
+		return tfstate.New(provider, "s3"), nil
 	case "azurekeyvault":
 		return azurekeyvault.New(provider), nil
 	}

vals.go

@@ -61,6 +61,8 @@ const (
 	ProviderFile             = "file"
 	ProviderGCPSecretManager = "gcpsecrets"
 	ProviderTFState          = "tfstate"
+	ProviderTFStateGS        = "tfstategs"
+	ProviderTFStateS3        = "tfstates3"
 	ProviderAzureKeyVault    = "azurekeyvault"
 )
 
@@ -158,7 +160,13 @@ func (r *Runtime) Eval(template map[string]interface{}) (map[string]interface{},
 			p := gcpsecrets.New(conf)
 			return p, nil
 		case ProviderTFState:
-			p := tfstate.New(conf)
+			p := tfstate.New(conf, "")
+			return p, nil
+		case ProviderTFStateGS:
+			p := tfstate.New(conf, "gs")
+			return p, nil
+		case ProviderTFStateS3:
+			p := tfstate.New(conf, "s3")
 			return p, nil
 		case ProviderAzureKeyVault:
 			p := azurekeyvault.New(conf)