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ARROW-12534: [C++][Gandiva] Implement LEFT and RIGHT functions on Gandiva for string input values

Impleented functions:

- LEFT (string, int):  Returns the x leftmost characters of giventext;
- RIGHT (string, int):  Returns the x rightmost characters of giventext;

Closes #10155 from jpedroantunes/feature/add-left-right-gandiva-functions and squashes the following commits:

23dbab8b4 <João Pedro> Fix identation on function string registry
7dc833c61 <João Pedro> Remove unnecessary validation
6ad707a44 <João Pedro> Change signature of left and right methods to consider argument types
4f33d851c <João Pedro> Add comments for the left and right functions
b0035522c <João Pedro> Change left function to iterate only once through the string
32697f6c7 <João Pedro> Change variables on left and right functions to be snake_case
6622a039e <João Pedro> Add missing space ident on string_ops.cc file
9aa250069 <João Pedro> Correct lint mistakes on left and right function iplementation
69cb2f739 <João Pedro> Add projector test for RIGHT string function
900bf7f58 <João Pedro> Add function registry for RIGHT string function
a7e542b51 <João Pedro> Add base implementation and tests for RIGHT function considering string input values
763547b1b <João Pedro> Add projector test for left string function
5870abb62 <João Pedro> Add function registry for left string function
3db95882a <João Pedro> Add base implementation and tests for left function considering string input values

Authored-by: João Pedro <joaop@simbioseventures.com>
Signed-off-by: Praveen <praveen@dremio.com>