feat: make stream_unsafe_cache_size a session variable

[lmatz]

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What's changed and what's your intention?

Right now, it is a global configuration variable. This PR makes a session variable.

1. In general cases, the optimal size of cache varies query by query and its data pattern. It is better if we can tune the setting for each query.
2. as a temporary workaround for store/cache only single value for first_value and last_value when the input is append-only #22293. Under append-only settings, some aggregation functions only need to store and cache 1 element, but right now we keep 10 elements in cache while storing all the elements. set stream_extreme_cache_size to 1 does not make sense for other queries.

Checklist

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Documentation

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Release note