JSONStore: fix last_updated serialization problem

[rkingsbury]

datetime objects serialize to str when using the orjson library, which breaks the last_updated functionality when a JSONStore is written and read back from disk. This PR fixes that problem by ensuring that the last_updated field, which is stored as a str, is properly deserialized into a datetime.

Also adds a new test and an additional test of the last_updated functionality in MontyStore.

[codecov]

Codecov Report

Base: 88.37% // Head: 88.38% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (3703012) compared to base (fe2a735).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
+ Coverage   88.37%   88.38%   +0.01%     
==========================================
  Files          41       41              
  Lines        3123     3126       +3     
==========================================
+ Hits         2760     2763       +3     
  Misses        363      363              

Impacted Files	Coverage Δ
src/maggma/stores/mongolike.py	88.07% <100.00%> (+0.09%)	⬆️

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[munrojm]

Thanks @rkingsbury! Happy for me to merge?

[rkingsbury]

Yep I'm good for you to merge!