Settings are not validated when snapshot restore is performed

[andrershov]

When snapshot restore is performed, setting Validator is not executed. So even if the setting does not pass validation, snapshot restore will succeed.
The easiest way to reproduce the issue is to inject dummy setting by installing Plugin during the test and break this setting validation before snapshot restore.

[elasticmachine]

Pinging @elastic/es-distributed

[DaveCTurner]

Copied from #50837: there is a different but related issue about the validation of the index settings supplied by the user when restoring a snapshot too:

POST /_snapshot/backing_repo/snap/_restore
{
  "index_settings": {
    "index.setting.misspelled": "blah",
    "index.frozen": true
  }
}
# 200 OK
# {
#   "accepted": true
# }

GET _settings
# 200 OK
# {
#   "original": {
#     "settings": {
#       "index": {
#         "provided_name": "original",
#         "frozen": "true",  // 💥 this setting is PrivateIndex
#         "number_of_shards": "1",
#         "uuid": "M5thzFiuTf6x-4_Fc82lMg",
#         "number_of_replicas": "0",
#         "version": {
#           "created": "8000099"
#         },
#         "creation_date": "1578592744159"
#       },
#       "archived": {
#         "index": {
#           "setting": {
#             "misspelled": "blah"  // 💥 this setting is unknown
#           }
#         }
#       }
#     }
#   }
# }

Ideally either of these settings should have caused the restore to fail.

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)