Adding component template substitutions to the simulate ingest API

[masseyke]

This adds the ability to pass substitute component template definitions to the simulate ingest API. These definitions can be used to change the pipeline(s) used for ingest, or to change the mappings used for validation. 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 two component templates. One provides a mapping enforces that the only field is "foo"
## and that field is a keyword. The other 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/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 "mappings_template" sets
## dynamic to true so that the introduction of the "bar" field doesn't cause problems.
## And the bar-pipeline is executed rather than the foo-pipeline because of the 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": {
    "mappings_template": {
      "template": {
        "mappings": {
          "dynamic": "strict",
          "properties": {
            "foo": {
              "type": "keyword"
            },
            "bar": {
              "type": "boolean"
            }
          }
        }
      }
    },
    "settings_template": {
      "template": {
        "settings": {
          "index": {
            "default_pipeline": "bar-pipeline"
          }
        }
      }
    }
  }
}

This builds on the work done in #112957 and #113063.

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Hi @masseyke, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[dakrone]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x