扁平化 JWE JSON 序列化语法

扁平的 JWE JSON 序列化语法基于通用语法，但是将其展平，针对单个收件人案例进行优化。

添加换行符以提高可读性

     {
      "protected":"<integrity-protected header contents>",
      "unprotected":<non-integrity-protected header contents>,
      "header":<more non-integrity-protected header contents>,
      "encrypted_key":"<encrypted key contents>",
      "aad":"<additional authenticated data contents>",
      "iv":"<initialization vector contents>",
      "ciphertext":"<ciphertext contents>",
      "tag":"<authentication tag contents>"
     }

例子

   {
     "protected": "eyJhbGciOiJBMTI4S1ciLCJraWQiOiI4MWIyMDk2NS04Mz
         MyLTQzZDktYTQ2OC04MjE2MGFkOTFhYzgiLCJlbmMiOiJBMTI4R0NNIn
         0",
     "encrypted_key": "4YiiQ_ZzH76TaIkJmYfRFgOV9MIpnx4X",
     "aad": "WyJ2Y2FyZCIsW1sidmVyc2lvbiIse30sInRleHQiLCI0LjAiXSxb
         ImZuIix7fSwidGV4dCIsIk1lcmlhZG9jIEJyYW5keWJ1Y2siXSxbIm4i
         LHt9LCJ0ZXh0IixbIkJyYW5keWJ1Y2siLCJNZXJpYWRvYyIsIk1yLiIs
         IiJdXSxbImJkYXkiLHt9LCJ0ZXh0IiwiVEEgMjk4MiJdLFsiZ2VuZGVy
         Iix7fSwidGV4dCIsIk0iXV1d",
     "iv": "veCx9ece2orS7c_N",
     "ciphertext": "Z_3cbr0k3bVM6N3oSNmHz7Lyf3iPppGf3Pj17wNZqteJ0
         Ui8p74SchQP8xygM1oFRWCNzeIa6s6BcEtp8qEFiqTUEyiNkOWDNoF14
         T_4NFqF-p2Mx8zkbKxI7oPK8KNarFbyxIDvICNqBLba-v3uzXBdB89fz
         OI-Lv4PjOFAQGHrgv1rjXAmKbgkft9cB4WeyZw8MldbBhc-V_KWZslrs
         LNygon_JJWd_ek6LQn5NRehvApqf9ZrxB4aq3FXBxOxCys35PhCdaggy
         2kfUfl2OkwKnWUbgXVD1C6HxLIlqHhCwXDG59weHrRDQeHyMRoBljoV3
         X_bUTJDnKBFOod7nLz-cj48JMx3SnCZTpbQAkFV",
     "tag": "vOaH_Rajnpy_3hOtqvZHRA"
   }