Separate SessionManager and SessionManagerLayer

[justmangoou]

Implementing the design from #11 .

This is not the final design so I expect any feedback. Also this could block (or be blocked by) #8

[codecov]

Codecov Report

Merging #15 (6c5e40e) into main (48f41bf) will decrease coverage by 3.85%.
Report is 1 commits behind head on main.
The diff coverage is 54.00%.

❗ Current head 6c5e40e differs from pull request most recent head 8963c18. Consider uploading reports for the commit 8963c18 to get more accurate results

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   79.35%   75.51%   -3.85%     
==========================================
  Files          13       13              
  Lines         431      441      +10     
==========================================
- Hits          342      333       -9     
- Misses         89      108      +19     

Files	Coverage Δ
src/lib.rs	25.00% <ø> (-8.34%)	⬇️
src/mongodb_store.rs	96.66% <87.50%> (-3.34%)	⬇️
src/service.rs	72.72% <83.33%> (+1.95%)	⬆️
src/cookie_config.rs	37.50% <0.00%> (-48.22%)	⬇️

[maxcountryman]

I think I missed from the previous discussion why this requires making the session manager a parameter of the layer.

[justmangoou]

I intentionally decided to separate SessionManager from SessionManagerLayer but I started thinking this is a bad decision

[justmangoou]

But if you're wondering, this

Edited design:

Allow construct SessionManager publicly.
Add SessionManager to State (idk if tower has this) and retrieve it from middleware.
Add option to allow SessionManager issues session automatically or not.