Support for transforms

[Cristigeo]

This PR is intended to add support for transforms (https://www.elastic.co/guide/en/elasticsearch/reference/current/transforms.html).

This is my first attempt to contribute to an open-source project; being new to this, I apologize in advance if I inadvertently break some rules/guidelines. Also, full disclaimer, Go is not my main programming language and I haven't done any previous coding for Terraform providers.

Feedback is welcome (obviously). While in its current state the code is functional (at least during the minimal tests I've performed), I'd like to point out a couple of design choices (in the current code) that may not be optimal:

• most configuration is done through dedicated resources block, but the pivot and latest props are handled as raw json content;
• the start/stop functionality is mapped through the enabled property. A different (better?) choice might have been a dedicated resource (such as elasticstack_elasticsearch_transform_state), because start/stop is a different API flow (not part of the usual CRUD).

Open points:

• drift detection
• support for import functionality
• improved documentation / acceptance test
• tests on different ES versions
• cleanup

Resolves #49

[cla-checker-service]

💚 CLA has been signed

[tobio]

the start/stop functionality is mapped through the enabled property. A different (better?) choice might have been a dedicated resource (such as elasticstack_elasticsearch_transform_state), because start/stop is a different API flow (not part of the usual CRUD).

Both approaches will work, and there doesn't seem to be an idiomatic approach here (some providers have a property on the resource, some have a separate resource). If you're happy with the property then we can start with that.

[tobio]

This is awesome! Some comments inline.

[tobio]

Also can we add a changelog entry here in this PR

[Cristigeo]

@tobio , thank you for the feedback, really appreciated. As you can tell, I'm still learning Go :) . I made a couple of pushes to fix most (if not all) of the issues. Not really sure about the "Resolve conversation" policy (who should be pushing those buttons), so I left that untouched.

[tobio]

Not really sure about the "Resolve conversation" policy

It's totally ok for you to hit those buttons. Personally I find that easier since it's clear that you've made changes to address that.

[tobio]

Excellent work! There's a lot of code in this contribution, thanks for taking the time to improve the provider :)