Implement languages endpoint

[Simply007]

Motivation

Fixes #95

Checklist

• [ ] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [ ] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

If manual testing is required, what are the steps?

[codecov[bot]]

Codecov Report

Merging #101 (141b68c) into master (3d0cd77) will decrease coverage by 0.21%. The diff coverage is 95.74%.

@@             Coverage Diff              @@
##             master     #101      +/-   ##
============================================
- Coverage     98.34%   98.13%   -0.22%     
- Complexity      180      194      +14     
============================================
  Files            19       22       +3     
  Lines           543      589      +46     
============================================
+ Hits            534      578      +44     
- Misses            9       11       +2     

Impacted Files	Coverage Δ
src/Kentico/Kontent/Delivery/LanguageFactory.php	90.90% <90.90%> (ø)
src/Kentico/Kontent/Delivery/DeliveryClient.php	94.35% <100.00%> (+0.71%)	:arrow_up:
...ntent/Delivery/Models/Languages/LanguageSystem.php	100.00% <100.00%> (ø)
...nt/Delivery/Models/Languages/LanguagesResponse.php	100.00% <100.00%> (ø)
src/Kentico/Kontent/Delivery/UrlBuilder.php	100.00% <100.00%> (ø)

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