Support multi-kinds separate with comma

This is the extension of ADR73 to support search across multi-kinds.

According to our feedback, quite a lots users prefer to code multi-kinds as a string separated by comma ',' instead of array of strings. This issue is to extend the coding or format of multi-kinds in search API, such as

{
   “kind”: “a:b:c:d,a:e:c:d”,
   "query": "well"
}

The proposal solution must be back-compatible.