ConversationSummaryMemory: Provide option to create the summary of the whole conversation

[bilgeyucel]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Introducing a new parameter to ConversationSummaryMemory to create the summary of the whole conversation. Currently, by default, it generates the summary of every three conversations but doesn't touch again the generated summaries

Describe alternatives you've considered
Increasing summary_frequency to a large number but this approach fills up the context window very quickly

Additional context
I find it useful if we foresee long chats (>~5 queries by human, setting summary_frequency=1), otherwise, it might be too much