Expose some internal structures for Crossplane provider

[ydubreuil]

This PR exposes some internal objects that are needed to build a Crossplane Elasticstack provider.

During initialization, the provider must instantiate a new ApiClient to use this Terraform provider as a library.

This is used like this in the Crossplane provider:

import (
...
	"github.com/elastic/terraform-provider-elasticstack/provider"
	"github.com/elastic/terraform-provider-elasticstack/xpprovider"
...
)

// TerraformSetupBuilder builds Terraform a terraform.SetupFn function which
// returns Terraform provider setup configuration
func TerraformSetupBuilder(version, providerSource, providerVersion string) terraform.SetupFn {
	return func(ctx context.Context, client client.Client, mg resource.Managed) (terraform.Setup, error) {
...
		apiClient, diag := xpprovider.NewApiClientFromFramework(ctx, fwpc, version)
		if diag.HasError() {
			return ps, errors.Errorf("%v", diag.Errors())
		}
		ps.Meta = apiClient
...

This patch is currently sitting in a branch from my fork that I use to build the Crossplane provider. It has been successfully used to create a PoC of a Crossplane Elasticstack provider.

[tobio]

Are you able to add an entry to CHANGELOG.md for this?

[ydubreuil]

@tobio I added an entry in the changelog