Add APM agent configuration

.windsurf/rules

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+* The project is a Terraform provider, written in Golang
+* The provider manages Elastic Stack resources
+* There is a mix of SDK, and plugin framework resources
+* New resources should be written with the Terraform plugin framework
+* Plugin framework resources should:
+    * Follow the example set in /internal/elasticsearch/security/system_user
+    * Be contained within their own module
+    * Have the main concepts split across their own files
+    * Include acceptance tests

CHANGELOG.md

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 ## [Unreleased]
 
+- Add `elasticstack_apm_agent_configuration` resource ([#1196](https://github.com/elastic/terraform-provider-elasticstack/pull/1196))
 - Add support for `timeslice_metric_indicator` in `elasticstack_kibana_slo` ([#1195](https://github.com/elastic/terraform-provider-elasticstack/pull/1195))
 - Add `elasticstack_elasticsearch_ingest_processor_reroute` data source ([#678](https://github.com/elastic/terraform-provider-elasticstack/issues/678))
 - Add support for `supports_agentless` to `elasticstack_fleet_agent_policy` ([#1197](https://github.com/elastic/terraform-provider-elasticstack/pull/1197))

docs/resources/apm_agent_configuration.md

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+---
+subcategory: "Kibana"
+layout: ""
+page_title: "Elasticstack: elasticstack_apm_agent_configuration Resource"
+description: |-
+  Creates or updates an APM agent configuration
+---
+
+# Resource: elasticstack_apm_agent_configuration
+
+Creates or updates an APM agent configuration. See https://www.elastic.co/docs/solutions/observability/apm/apm-agent-central-configuration
+
+## Example Usage
+
+```terraform
+provider "elasticstack" {
+  elasticsearch {}
+}
+
+resource "elasticstack_apm_agent_configuration" "test_config" {
+  service_name        = "my-service"
+  service_environment = "production"
+  agent_name          = "go"
+  settings = {
+    "transaction_sample_rate" = "0.5"
+    "capture_body"            = "all"
+  }
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `service_name` (String) The name of the service.
+- `settings` (Map of String) Agent configuration settings.
+
+### Optional
+
+- `agent_name` (String) The agent name is used by the UI to determine which settings to display.
+- `service_environment` (String) The environment of the service.
+
+### Read-Only
+
+- `id` (String) Internal identifier of the resource.
+
+## Import
+
+Import is supported using the following syntax:
+
+```shell
+terraform import elasticstack_apm_agent_configuration.test_configuration my-service:production
+```

examples/resources/elasticstack_apm_agent_configuration/import.sh

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+terraform import elasticstack_apm_agent_configuration.test_configuration my-service:production

examples/resources/elasticstack_apm_agent_configuration/resource.tf

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+provider "elasticstack" {
+  elasticsearch {}
+}
+
+resource "elasticstack_apm_agent_configuration" "test_config" {
+  service_name        = "my-service"
+  service_environment = "production"
+  agent_name          = "go"
+  settings = {
+    "transaction_sample_rate" = "0.5"
+    "capture_body"            = "all"
+  }
+}