FlatFeedCore tweaks

[GroovinChip]

This PR makes some minor tweaks to FlatFeedCore

• Allows the passing through of ScrollPhysics to FlatFeedCore. This allows for more flexible options in more complex layouts.
• Use ListView.separated

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

[codecov]

Codecov Report

Merging #171 (a306e5b) into support-extraData-null-value (e4947b3) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@                       Coverage Diff                        @@
##           support-extraData-null-value     #171      +/-   ##
================================================================
- Coverage                         76.38%   76.32%   -0.06%     
================================================================
  Files                                87       87              
  Lines                              2638     2640       +2     
================================================================
  Hits                               2015     2015              
- Misses                              623      625       +2     

Impacted Files	Coverage Δ
...ream_feed_flutter_core/lib/src/flat_feed_core.dart	64.86% <0.00%> (-3.71%)	⬇️

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