🔒️(backend) add environment variables for PKCE settings

Defaulting to off for now to keep compatibility.

src/backend/meet/settings.py

@@ -430,6 +430,17 @@ class Base(Configuration):
     OIDC_RP_SCOPES = values.Value(
         "openid email", environ_name="OIDC_RP_SCOPES", environ_prefix=None
     )
+    OIDC_USE_PKCE = values.BooleanValue(
+        default=False, environ_name="OIDC_USE_PKCE", environ_prefix=None
+    )
+    OIDC_PKCE_CODE_CHALLENGE_METHOD = values.Value(
+        default="S256",
+        environ_name="OIDC_PKCE_CODE_CHALLENGE_METHOD",
+        environ_prefix=None,
+    )
+    OIDC_PKCE_CODE_VERIFIER_SIZE = values.IntegerValue(
+        default=64, environ_name="OIDC_PKCE_CODE_VERIFIER_SIZE", environ_prefix=None
+    )
     LOGIN_REDIRECT_URL = values.Value(
         None, environ_name="LOGIN_REDIRECT_URL", environ_prefix=None
     )