feat(coderd): add support for external agents to API's and provisioner (#19286)

This pull request introduces support for external workspace management, allowing users to register and manage workspaces that are provisioned and managed outside of the Coder.

Depends on: https://github.com/coder/terraform-provider-coder/pull/424

* GET /api/v2/init-script - Gets the agent initialization script
  * By default, it returns a script for Linux (amd64), but with query parameters (os and arch) you can get the init script for different platforms
* GET /api/v2/workspaces/{workspace}/external-agent/{agent}/credentials - Gets credentials for an external agent **(enterprise)**
* Updated queries to filter workspaces/templates by the has_external_agent field

docs/reference/api/initscript.md

@@ -0,0 +1,26 @@
+# InitScript
+
+## Get agent init script
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X GET http://coder-server:8080/api/v2/init-script/{os}/{arch}
+
+```
+
+`GET /init-script/{os}/{arch}`
+
+### Parameters
+
+| Name   | In   | Type   | Required | Description      |
+|--------|------|--------|----------|------------------|
+| `os`   | path | string | true     | Operating system |
+| `arch` | path | string | true     | Architecture     |
+
+### Responses
+
+| Status | Meaning                                                 | Description | Schema |
+|--------|---------------------------------------------------------|-------------|--------|
+| 200    | [OK](https://tools.ietf.org/html/rfc7231#section-6.3.1) | Success     |        |