VAULT-25710: Audit - enforce header formatter requirement in EntryFormatter (#26239)

* Check Enterprise unseal order for audit funcs, enforce header formatter in audit entry formatter node
* ApplyConfig return empty headers (but never nil) when nil/empty supplied
* Add NoopHeaderFormatter and remove builtin audit testHeaderFormatters

audit/options.go

@@ -5,7 +5,6 @@ package audit
 
 import (
 	"errors"
-	"reflect"
 	"strings"
 	"time"
 )
@@ -15,16 +14,15 @@ type Option func(*options) error
 
 // options are used to represent configuration for a audit related nodes.
 type options struct {
-	withID              string
-	withNow             time.Time
-	withSubtype         subtype
-	withFormat          format
-	withPrefix          string
-	withRaw             bool
-	withElision         bool
-	withOmitTime        bool
-	withHMACAccessor    bool
-	withHeaderFormatter HeaderFormatter
+	withID           string
+	withNow          time.Time
+	withSubtype      subtype
+	withFormat       format
+	withPrefix       string
+	withRaw          bool
+	withElision      bool
+	withOmitTime     bool
+	withHMACAccessor bool
 }
 
 // getDefaultOptions returns options with their default values.
@@ -163,15 +161,3 @@ func WithHMACAccessor(h bool) Option {
 		return nil
 	}
 }
-
-// WithHeaderFormatter provides an Option to supply a HeaderFormatter.
-// If the HeaderFormatter interface supplied is nil (type or value), the option will not be applied.
-func WithHeaderFormatter(f HeaderFormatter) Option {
-	return func(o *options) error {
-		if f != nil && !reflect.ValueOf(f).IsNil() {
-			o.withHeaderFormatter = f
-		}
-
-		return nil
-	}
-}