Add a method to get the changes of a specific commit id

[AurelienFT]

This PR adds a methods get_changes() that allows you to get the changes committed at a certain id.

I have added documentation and tests on it in the PR if it's not clear do not hesitate to ask anything :)

[codecov]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (d2dd35d) 57.84% compared to head (a63fc25) 90.71%.
Report is 7 commits behind head on oss.

Files	Patch %	Lines
src/error.rs	0.00%	8 Missing ⚠️
src/trie/merkle_tree.rs	94.24%	8 Missing ⚠️
src/trie/path.rs	90.24%	8 Missing ⚠️
src/key_value_db.rs	84.84%	5 Missing ⚠️
src/trie/trie_db.rs	77.27%	5 Missing ⚠️
src/databases/hashmap_db.rs	0.00%	1 Missing ⚠️
src/databases/rocks_db.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              oss      #13       +/-   ##
===========================================
+ Coverage   57.84%   90.71%   +32.86%     
===========================================
  Files          11       17        +6     
  Lines         344     2154     +1810     
===========================================
+ Hits          199     1954     +1755     
- Misses        145      200       +55     

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