Added graph_coloring algorithm to backtracking

[Rimeeeeee]

Description

Graph-coloring algorithm implemented through backtracking .The process colors graph in such a way that no 2 adjacent vertices are of same color.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.06%. Comparing base (f8096d2) to head (77eaafb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #759      +/-   ##
==========================================
+ Coverage   95.04%   95.06%   +0.02%     
==========================================
  Files         305      306       +1     
  Lines       22674    22780     +106     
==========================================
+ Hits        21550    21656     +106     
  Misses       1124     1124              

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[vil02]

@Rimeeeeee I think this PR is a duplicate of #737. Could you please verify that?

[Rimeeeeee]

@Rimeeeeee I think this PR is a duplicate of #737. Could you please verify that?

@vil02 I didn't notice PR #737. The implementation is diff.