Add serverless project resources

[tobio]

Description

Currently only includes elasticsearch_project resources. IMO we should include at least all project types, also likely worthwhile including a serverless_region data source, and maybe project data sources since they should be simple to add re-using the resource read code.

This PR also include the terraform code generators, currently configured to generate the serverless resource schema and openapi client.

How Has This Been Tested?

Manually, acceptance tests. I need to add some more unit tests here.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

I need to add more tests and docs here.

[dimuon]

@tobio , make gen adds the license header to a bunch of files.

[dimuon]

Does it make sense to document updating serveless OpenAPI specs and mention where to look for the updated spec.json?

[dimuon]

Ops, my bad, I mean where to look for the updated serverless-project-api-dereferenced.yml.

[dimuon]

@tobio , I completed the review (though I skim through security_test.go and observability_test.go which look similar to elasticsearch_test.go)

[dimuon]

👍 LGTM