Allow passing a messagesMapper prop when instantiating mastra agent

[maxkorp]

For example, this allows you to take an array slice to provide only a single message (most recent) to the agent when using memory

I'm not entirely convinced I like this solution. It's possible we just always want to filter to the latest message if memory is in play. In that case, we can just automatically filter, but I was unsure if we wanted to always do that, or if there are cases where more than one message may be sent and need to be pushed in.