Adds set/save to ParseConfig

[montymxb]

This adds set/save functionality to ParseConfig, which interacts with the _GlobalConfig collection server-side (of which there should only ever be one instance). Associated tests are added as well.

You can set/save as follows.

// gets the current parse config
$config = new ParseConfig();

// sets a new key/value pair in the config
$config->set('key', 'value');

// saves the current config back to the server
$config->save(); // internally uses the masterKey to save

[codecov]

Codecov Report

Merging #371 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   98.93%   98.94%   +<.01%     
==========================================
  Files          36       36              
  Lines        3389     3403      +14     
==========================================
+ Hits         3353     3367      +14     
  Misses         36       36

Impacted Files	Coverage Δ
src/Parse/ParseConfig.php	100% <100%> (ø)	⬆️

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