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"
      }
    }
  )

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• 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?

[konalegi]

@TakuyaKurimoto Sorry, was extremely busy. Is this PR still relevant?

[TakuyaKurimoto]

@konalegi
Thanks for getting back to me. The problem is still there, so I'd appreciate it if you could apply this patch.

[konalegi]

Cool, can you rebase and push latest changes so I can run CI?

[TakuyaKurimoto]

@konalegi
rebased! Please run CI.

[konalegi]

Everything is passing, let me get familiar with the topic, and I'll merge it.

[TakuyaKurimoto]

Thank you!

[konalegi]

@TakuyaKurimoto looks good 👍 Although could you please add a couple of lines into documentation about this feature, no extensive doc is needed, just an example + reference to elastic doc, so people know that this feature exists.
Thanks!

[TakuyaKurimoto]

@konalegi
Sorry, but which file are you referring to when you say "documentation"? Is it CHANGELOG.md?

[konalegi]

@TakuyaKurimoto Actually I'm about README.md

[TakuyaKurimoto]

@konalegi
I understand. I've added the document, so please check it.