feat: support auto-prefix for nested configs and custom prefix separator

[Rancbar]

Title:
Add AutoPrefix and PrefixSeparator support for improved nested field prefix handling

Description:

This PR introduces two new features to enhance prefix handling during environment variable parsing:

AutoPrefix (bool): Automatically applies the struct field key as the prefix when no explicit envPrefix is defined.

PrefixSeparator (string): Defines a custom separator in combining key for nested key lookups (e.g., _ results in FOO_TEST).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8917d78) to head (1381ff3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #367   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines          622       644   +22     
=========================================
+ Hits           622       644   +22     

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