cmd/{derp,derpprobe},prober,derp: add mesh support to derpprobe (#15414)

Add mesh key support to derpprobe for
probing derpers with verify set to true.

Move MeshKey checking to central point for code reuse.

Fix a bad error fmt msg.

Fixes tailscale/corp#27294
Fixes tailscale/corp#25756

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

types/key/derp.go

@@ -6,6 +6,7 @@ package key
 import (
 	"crypto/subtle"
 	"encoding/hex"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"strings"
@@ -23,6 +24,27 @@ type DERPMesh struct {
 	k [32]byte             // 64-digit hexadecimal numbers fit in 32 bytes
 }
 
+// MarshalJSON implements the [encoding/json.Marshaler] interface.
+func (k DERPMesh) MarshalJSON() ([]byte, error) {
+	return json.Marshal(k.String())
+}
+
+// UnmarshalJSON implements the [encoding/json.Unmarshaler] interface.
+func (k *DERPMesh) UnmarshalJSON(data []byte) error {
+	var s string
+	json.Unmarshal(data, &s)
+
+	if hex.DecodedLen(len(s)) != len(k.k) {
+		return fmt.Errorf("types/key/derp: cannot unmarshal, incorrect size mesh key len: %d, must be %d, %w", hex.DecodedLen(len(s)), len(k.k), ErrInvalidMeshKey)
+	}
+	_, err := hex.Decode(k.k[:], []byte(s))
+	if err != nil {
+		return fmt.Errorf("types/key/derp: cannot unmarshal, invalid mesh key: %w", err)
+	}
+
+	return nil
+}
+
 // DERPMeshFromRaw32 parses a 32-byte raw value as a DERP mesh key.
 func DERPMeshFromRaw32(raw mem.RO) DERPMesh {
 	if raw.Len() != 32 {