Implement substr command

[danielealbano]

The PR implements the SUBSTR command, although it's deprecated it's still available.

The SUBSTR command is a clone of the GETRANGE command, tests have been duplicated and adapted as needed.

[codecov]

Codecov Report

Base: 80.35% // Head: 80.40% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (dc947e8) compared to base (00b8b28).
Patch coverage: 93.10% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
+ Coverage   80.35%   80.40%   +0.05%     
==========================================
  Files         135      136       +1     
  Lines        8062     8091      +29     
==========================================
+ Hits         6478     6505      +27     
- Misses       1584     1586       +2     

Impacted Files	Coverage Δ
...module/redis/command/module_redis_command_substr.c	93.10% <93.10%> (ø)

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