Add runtime_mappings support

[TakuyaKurimoto]

Hello. One time I wanted to use runtime_mappings, but in chewy I had to write the following code because chewy does not support runtime_mappings .

q = HogeIndex.query({}).render
q[:body][:runtime_mappings] = {"field1"=>{type: 'keyword', :script=>{:lang=>"painless", :source=>"emit('some script here')"}}}
HogeIndex.client.search(body: q[:body])

This is a bit inconvenient, so I was wondering if you could support runtime_mappings in chewy as well?

If this code is merged, it will be possible to do something like the code below

CitiesIndex
  .runtime_mappings(
    field1: {
      type: 'keyword',
      :script=>{
        :lang=>"painless",
        :source=>"emit('some script here')"
      }
    }
  )
  .aggs(
    field1: {
      terms: {
        field: "field1"
      }
    }
  )

---

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[konalegi]

@TakuyaKurimoto Thanks for the PR! unfortunately I'm not familiar with the runtime mappings features, let me learn it and get back to you.

[TakuyaKurimoto]

OK! I'm sorry for bothering you.

[TakuyaKurimoto]

@konalegi sorry for bothering you. Any updates on that one?