…ic#114128)

This adds support for a new `index_template_substitutions` field to the
body of an ingest simulate API request. These substitutions can be used
to change the pipeline(s) used for ingest, or to change the mappings
used for validation. It is similar to the
`component_template_substitutions` added in elastic#113276. Here is an example
that shows both of those usages working together:

```
## First, add a couple of pipelines that set a field to a boolean:
PUT /_ingest/pipeline/foo-pipeline?pretty
{
  "processors": [
    {
      "set": {
        "field": "foo",
        "value": true
      }
    }
  ]
}

PUT /_ingest/pipeline/bar-pipeline?pretty
{
  "processors": [
    {
      "set": {
        "field": "bar",
        "value": true
      }
    }
  ]
}

## Now, create three component templates. One provides a mapping enforces that the only field is "foo"
## and that field is a keyword. The next is similar, but adds a `bar` field. The final one provides a setting
## that makes "foo-pipeline" the default pipeline.
## Remember that the "foo-pipeline" sets the "foo" field to a boolean, so using both of these templates
## together would cause a validation exception. These could be in the same template, but are provided
## separately just so that later we can show how multiple templates can be overridden.
PUT _component_template/mappings_template
{
  "template": {
    "mappings": {
      "dynamic": "strict",
      "properties": {
        "foo": {
          "type": "keyword"
        }
      }
    }
  }
}

PUT _component_template/mappings_template_with_bar
{
    "template": {
      "mappings": {
        "dynamic": "strict",
        "properties": {
          "foo": {
            "type": "keyword"
          },
          "bar": {
            "type": "boolean"
          }
        }
      }
    }
}

PUT _component_template/settings_template
{
  "template": {
    "settings": {
      "index": {
        "default_pipeline": "foo-pipeline"
      }
    }
  }
}

## Here we create an index template  pulling in both of the component templates above
PUT _index_template/template_1
{
  "index_patterns": ["foo*"],
  "composed_of": ["mappings_template", "settings_template"]
}

## We can index a document here to create the index, or not. Either way the simulate call ought to work the same
POST foo-1/_doc
{
  "foo": "FOO"
}

## This will not blow up with validation exceptions because the substitute "index_template_substitutions"
## uses `mappings_template_with_bar`, which adds the bar field.
## And the bar-pipeline is executed rather than the foo-pipeline because the substitute
## "index_template_substitutions" uses a substitute `settings_template`, so the value of "foo"
## does not get set to an invalid type.
POST _ingest/_simulate?pretty&index=foo-1
{
  "docs": [
    {
      "_id": "asdf",
      "_source": {
        "foo": "foo",
        "bar": "bar"
      }
    }
  ],
  "component_template_substitutions": {
    "settings_template": {
      "template": {
        "settings": {
          "index": {
            "default_pipeline": "bar-pipeline"
          }
        }
      }
    }
  },
  "index_template_substitutions": {
    "template_1": {
      "index_patterns": ["foo*"],
      "composed_of": ["mappings_template_with_bar", "settings_template"]
    }
  }
}
```