states: ParseDeposedKey function

This is a helper for when we're accepting a deposed key from outside of
Terraform, such as in a plan or state file. It doesn't really "parse"
the string in the usual sense -- the result is always just the input
verbatim if successful -- but it's named that way for consistency with
how we usually name functions like this.

internal/states/resource.go

@@ -4,8 +4,10 @@
 package states
 
 import (
+	"encoding/hex"
 	"fmt"
 	"math/rand"
+	"strings"
 	"time"
 
 	"github.com/hashicorp/terraform/internal/addrs"
@@ -189,6 +191,22 @@ func NewDeposedKey() DeposedKey {
 	return DeposedKey(fmt.Sprintf("%08x", v))
 }
 
+// ParseDeposedKey parses a string that is expected to be a deposed key,
+// returning an error if it doesn't conform to the expected syntax.
+func ParseDeposedKey(raw string) (DeposedKey, error) {
+	if len(raw) != 8 {
+		return "00000000", fmt.Errorf("must be eight hexadecimal digits")
+	}
+	if raw != strings.ToLower(raw) {
+		return "00000000", fmt.Errorf("must use lowercase hex digits")
+	}
+	_, err := hex.DecodeString(raw)
+	if err != nil {
+		return "00000000", fmt.Errorf("must be eight hexadecimal digits")
+	}
+	return DeposedKey(raw), nil
+}
+
 func (k DeposedKey) String() string {
 	return string(k)
 }