fix: convert last checkpoint json keys to camelCase

[feniljain]

Description

While storing _last_checkpoint delta-rs does not convert json keys to camel case, but protocol mentions they should be in camelCase, fix exactly that :)

Related Issue(s)

Fixes delta-incubator/delta-kernel-rs#326

Documentation

Issue description is nicely documented :)

Output screenshots:

Tested with local crate copy and created a table

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.41%. Comparing base (b3b23be) to head (3411eb6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2889      +/-   ##
==========================================
+ Coverage   72.37%   72.41%   +0.04%     
==========================================
  Files         133      133              
  Lines       40238    40253      +15     
  Branches    40238    40253      +15     
==========================================
+ Hits        29124    29151      +27     
+ Misses       9239     9237       -2     
+ Partials     1875     1865      -10     

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[ion-elgreco]

@feniljain can you add a test?

[feniljain]

Hey, I have added a small test. It's hacky, and I am not sure if you are looking for a different type of test. If yes, do help me out by pointing me to it 😅