Enable getting Stomp authentication credentials from the environment

src/blueapi/config.py

@@ -1,9 +1,10 @@
+import os
 from enum import Enum
 from pathlib import Path
 from typing import Any, Dict, Generic, Literal, Mapping, Optional, Type, TypeVar, Union
 
 import yaml
-from pydantic import BaseModel, Field, ValidationError, parse_obj_as
+from pydantic import BaseModel, Field, ValidationError, parse_obj_as, validator
 
 from blueapi.utils import BlueapiBaseModel, InvalidConfigError
 
@@ -29,6 +30,12 @@ class BasicAuthentication(BaseModel):
     username: str = "guest"
     passcode: str = "guest"
 
+    @validator("username", "passcode")
+    def get_passcode_from_env(cls, v: str):
+        if v.startswith("${") and v.endswith("}"):
+            return os.environ[v.removeprefix("${").removesuffix("}").upper()]
+        return v
+
 
 class StompConfig(BaseModel):
     """