feat(rules): add support for detecting high entropy strings in composite literals

[oittaa]

Currently gosec doesn't detect something like this

package main

type DBConfig struct {
        Password string
}

func main() {
        _ = DBConfig{
                Password: "f62e5bcda4fae4f82370da0c6f20697b8f8447ef",
        }
}

I think this is also a step towards implementing #1416

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.77%. Comparing base (1216c9b) to head (50a0054).
⚠️ Report is 149 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
- Coverage   68.49%   64.77%   -3.73%     
==========================================
  Files          75       78       +3     
  Lines        4384     4761     +377     
==========================================
+ Hits         3003     3084      +81     
- Misses       1233     1530     +297     
+ Partials      148      147       -1     

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