Defines how long we retain persisted data that contains
sensitive info.
Values:
RETENTION_STRATEGY_UNSPECIFIED (0):
Retains the persisted data with Dialogflow's
internal default 365d TTLs.
REMOVE_AFTER_CONVERSATION (1):
Removes data when the conversation ends. If there is no
[Conversation][] explicitly established, a default
conversation ends when the corresponding Dialogflow session
ends.