Added #isSuccessful() methods to result types

[dxslly]

This is a helper method that allows client developers to check is a request was successful without knowledge of the result codes. This helps to standardize interactions with the CredntialClient API for each requests.

e.g. if (result.isSuccessful()) {} vs if (ResultObject.CODE_SUCCESSFUL == result.getResultCode()) {}

[codecov-io]

Codecov Report

Merging #87 into master will decrease coverage by 0.11%. The diff coverage is 0%.

@@             Coverage Diff              @@
##             master      #87      +/-   ##
============================================
- Coverage     71.64%   71.53%   -0.12%     
  Complexity      454      454              
============================================
  Files            62       62              
  Lines          2536     2540       +4     
  Branches        242      242              
============================================
  Hits           1817     1817              
- Misses          652      656       +4     
  Partials         67       67

Impacted Files	Coverage Δ	Complexity Δ
...va/org/openyolo/protocol/CredentialSaveResult.java	90.74% <0%> (-1.72%)	8 <0> (ø)
.../org/openyolo/protocol/CredentialDeleteResult.java	98.21% <0%> (-1.79%)	9 <0> (ø)
...rg/openyolo/protocol/CredentialRetrieveResult.java	90.32% <0%> (-1.49%)	10 <0> (ø)
...java/org/openyolo/protocol/HintRetrieveResult.java	88.7% <0%> (-1.46%)	9 <0> (ø)

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