add support for pinning mfs

[aschmahmann]

Needed for ipfs/kubo#7798
Part of ipfs/kubo#7559 / ipfs/ipfs-gui#91

[petar]

LGTM

[lidel]

lgtm.

Note that even tho we use json:",omitempty" for Policies and MFS structs, theRemotePinningServiceMFSPolicy.Enable is bool, so ipfs pin remote service add will create an explicit Policies.MFS:

     "RemoteServices": {
      "testsrv": {
        "API": {
          "Endpoint": "https://testsrv.example.com",
           "Key": "secret"
         },
         "Policies": {
           "MFS": {
             "Enable": false
           }
         }
       }
     }