Add TTL to runs stored in conversational memory

[igor-2lemetry]

• I have read and agree to the contributing guidelines.

Is your feature request related to a problem? Please describe.
It would be great to exclude from prompt stack too old runs.

Describe the solution you'd like
Possible it should be done by adding ttl attribute in ConversationMemory and some timestamp for each run.

Describe alternatives you've considered
Alternatives are: pruning by max_runs or tokens_left.

Additional context
No additional context.

[collindutter]

@igor-2lemetry I think this is best suited for the database to handle, what do you think?