Add IndexTemplate field under elasticsearch field

[mrodm]

According to the package-spec data_stream manifest definition, the structure under elasticsearch field should be:

elasticsearch:
  index_template:
    mappings:
      ...
    ingest_pipeline:
      name:

Currently, DataStreamManifest definition does not include index_template field https://github.com/elastic/elastic-package/blob/019ab37/internal/packages/packages.go#L141-L145

This PR adds the index_template key to be compliant with the package-spec. Currently, there is no package defining/using this key in the integrations repository.

This addition is also required if at some point mappings need to be taken into account too.

Relates #1018

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[jsoriano]

This PR adds the index_template key to be compliant with the package-spec. Currently, there is no package defining/using this key in the integrations repository.

This is used by the APM package, that is not in the integrations repository, see here for example: https://github.com/elastic/apm-server/blob/a705bd788cba5f78d82844dab8e3485745844010/apmpackage/apm/data_stream/app_metrics/manifest.yml#L7

[mrodm]

This PR adds the index_template key to be compliant with the package-spec. Currently, there is no package defining/using this key in the integrations repository.

This is used by the APM package, that is not in the integrations repository, see here for example: https://github.com/elastic/apm-server/blob/a705bd788cba5f78d82844dab8e3485745844010/apmpackage/apm/data_stream/app_metrics/manifest.yml#L7

Right, apm package and also the endpoint package use this key (https://github.com/elastic/endpoint-package/search?q=index_template&type=code). In both repositories, they are using the structure of the manifest in data_streams as defined in package-spec, so that's correct.

elastic-package uses this information to retrieve the pipeline name (elasticsearch.index_template.ingest_pipeline.name) in this function:
https://github.com/elastic/elastic-package/blob/main/internal/packages/packages.go#L257
I tried to look for usages of ingest_pipeline in packages, but I didn't find any. I would say this change is needed so elastic-package gets that field value according to what package-spec defines. If not, I think it will always use the default value.

@jsoriano do you think this change could cause any issue?