Added API server context.

[AlexSkrypnyk]

/**
 * API test server to return queued responses to HTTP requests.
 *
 * The requests and responses can be enqueued via `/admin/*` endpoints.
 *
 * Supported endpoints:
 * - GET `/admin/status`: Check the server status.
 *   > HTTP/1.1 200 OK
 *   > X-Received-Requests: 0
 *   > X-Queued-Responses: 0
 *
 * - GET `/admin/requests`: Get the received requests.
 *   > HTTP/1.1 200 OK
 *   > X-Received-Requests: 1
 *   > X-Queued-Responses: 0
 *   > Content-Type: application/json
 *   > [{'http_method': 'GET', 'uri': '/', 'headers': {}, 'body': 'string'}]
 *
 * - DELETE `/admin/requests`: Delete all received requests.
 *   > HTTP/1.1 200 OK
 *   > X-Received-Requests: 0
 *   > X-Queued-Responses: 0
 *
 * - GET `/admin/responses`: Get the queued responses.
 *   > HTTP/1.1 200 OK
 *   > X-Received-Requests: 0
 *   > X-Queued-Responses: 1
 *   > Content-Type: application/json
 *   > [{'status': 200, 'reason': 'OK', 'headers': {}, 'body': '' }]
 *
 * - DELETE `/admin/responses`: Delete all queued responses.
 *   > HTTP/1.1 200 OK
 *   > X-Received-Requests: 0
 *   > X-Queued-Responses: 0
 *
 * - PUT `/admin/responses`: Enqueue responses.
 *   > HTTP/1.1 201 Created
 *   > X-Received-Requests: 0
 *   > X-Queued-Responses: 1
 *   > Content-Type: application/json
 *   > [{'status': 200, 'reason': 'OK', 'headers': {}, 'body': '' }, {'status': 404, 'reason': 'Not found', 'headers': {}, 'body': '' }]
 *
 * This class is intended to be lightweight and limited in functionality.
 *
 * @phpcs:disable Drupal.Classes.ClassFileName.NoMatch
 */