refactor(parameters): Contistently reference env

Use the defined constants to look up service name environment varialbe NOTE: This does not default to `service_undefined`

aws_lambda_powertools/utilities/parameters/appconfig.py

@@ -10,6 +10,7 @@
 import boto3
 from botocore.config import Config
 
+from ...shared import constants
 from .base import DEFAULT_PROVIDERS, BaseProvider
 
 CLIENT_ID = str(uuid4())
@@ -33,6 +34,7 @@ class AppConfigProvider(BaseProvider):
  **Retrieves the latest configuration value from App Config**
 
  >>> from aws_lambda_powertools.utilities import parameters
+ >>>
  >>> appconf_provider = parameters.AppConfigProvider(environment="my_env", application="my_app")
  >>>
  >>> value : bytes = appconf_provider.get("my_conf")
@@ -66,7 +68,7 @@ def __init__(
 
  config = config or Config()
  self.client = boto3.client("appconfig", config=config)
- self.application = application or os.getenv("POWERTOOLS_SERVICE_NAME") or "application_undefined"
+ self.application = application or os.getenv(constants.SERVICE_NAME_ENV, "service_undefined")
  self.environment = environment
  self.current_version = ""