chore(testing): added TF templates for tests

[vmyroslav]

This PR introduces a template-based testing pkg that simplifies writing and maintaining Terraform provider acceptance tests. It provides a declarative and composable approach to constructing test configurations.

Key Features

• Replaces string templates with structured templates
• Pre-registered templates for all provider resources
• Composable test configurations
• Dynamic template modifications during tests

Benefits

• Easy dynamic modifications during tests
• Reusable templates across tests
• Built-in validation of resource configurations

Usage Example

// Using the new template-based framework
func TestExpectedBehaviour(t *testing.T) {
    // Get template set with all provider resources pre-registered
    templateSet := GetTemplateSet(t)

    // Add custom template if needed
    templateSet.AddTemplate("aiven_kafka", `
resource "aiven_kafka" "bar" {
  project                 = data.aiven_project.foo.project
  cloud_name              = "google-europe-west1"
  plan                    = "startup-2"
  service_name            = "{{ .service_name }}"
  maintenance_window_dow  = "monday"
  maintenance_window_time = "10:00:00"
}`)

    // Add external provider templates
    templateSet.AddTemplate("gcp_provider", `
provider "google" {
    project = "{{ .gcp_project }}"
    region  = "{{ .gcp_region }}"
}`)

    // Build configuration using fluent interface
    builder := templateSet.NewBuilder().
       // Add data source with configuration
       AddDataSource("aiven_project", map[string]any{
          "resource_name": "foo",
          "project":       "test_project",
       }).
       // Add resources with different names
       AddResource("aiven_kafka_quota", map[string]any{
          "resource_name":      "example_quota_1",
          "project":            "test_project",
          "service_name":       "test-service",
          "user":               "test_user",
          "client_id":          "test_client_id",
          "consumer_byte_rate": 1000,
       }).
       AddResource("aiven_kafka_quota", map[string]any{
          "resource_name":      "example_quota_2",
          "project":            Reference("data.aiven_project.foo.project"), // Reference existing resource
          "service_name":       "test_service_2",
          "user":               "test_user_2",
          "client_id":          "test_client_id_2",
          "consumer_byte_rate": 1000,
       }).
       // Add custom templates
       Add("aiven_kafka", map[string]any{"service_name": "test-service"}).
       Add("gcp_provider", map[string]any{
          "gcp_project": "test-project", 
          "gcp_region": "us-central1",
       })

    // Render the full configuration
    str := builder.MustRender(t)
    assert.NotEmpty(t, str)

    // Modify configuration by removing resource
    builder.RemoveByResourceName("example_quota_1")

    // Render updated configuration
    nextStep := builder.MustRender(t)
    assert.NotEmpty(t, nextStep)
}