Exporting CubejsApi class

[Spittal]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made

I have a specific authorization use case that I would like to add to the CubejsApi class. Without access to the class I have to modify the instantiated object which doesn't feel as clean or scalable.

This change just adds the CubejsApi class to the named exports so that more experienced developers can extend the API.

[codecov]

Codecov Report

Merging #2773 (d44bf98) into master (4dc55b4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2773   +/-   ##
=======================================
  Coverage   56.83%   56.83%           
=======================================
  Files         115      115           
  Lines        8792     8792           
  Branches     1871     1871           
=======================================
  Hits         4997     4997           
  Misses       3457     3457           
  Partials      338      338           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 00e42b8...d44bf98. Read the comment docs.