Feat: conversation variable & variable assigner node (#7222)

Signed-off-by: -LAN- <laipz8200@outlook.com>
Co-authored-by: Joel <iamjoel007@gmail.com>
Co-authored-by: -LAN- <laipz8200@outlook.com>

api/configs/app_config.py

@@ -12,19 +12,14 @@ from configs.packaging import PackagingInfo
 class DifyConfig(
     # Packaging info
     PackagingInfo,
-
     # Deployment configs
     DeploymentConfig,
-
     # Feature configs
     FeatureConfig,
-
     # Middleware configs
     MiddlewareConfig,
-
     # Extra service configs
     ExtraServiceConfig,
-
     # Enterprise feature configs
     # **Before using, please contact business@dify.ai by email to inquire about licensing matters.**
     EnterpriseFeatureConfig,
@@ -36,7 +31,6 @@ class DifyConfig(
         env_file='.env',
         env_file_encoding='utf-8',
         frozen=True,
-
         # ignore extra attributes
         extra='ignore',
     )
@@ -67,3 +61,5 @@ class DifyConfig(
     SSRF_PROXY_HTTPS_URL: str | None = None
 
     MODERATION_BUFFER_SIZE: int = Field(default=300, description='The buffer size for moderation.')
+
+    MAX_VARIABLE_SIZE: int = Field(default=5 * 1024, description='The maximum size of a variable. default is 5KB.')