Allow composing templates from partial templates

[endorama]

Originally proposed by @ruflin in https://github.com/elastic/elastic-integration-corpus-generator-tool/pull/43/files#r1091814491:

For later: Would be nice if a template could reference a template for these generic bits that show up each template.

The feature request is to allow for templates to reuse shared template sections. This may be achieved by allowing partial templates than are composed into the final template.