Add support for auth using HTTP Authorization request header

[dtatarkin]

Prior to April 2020, API keys would be included as a part of the API request body or within the request URL as a parameter. Braze now has updated the way in which we read API keys. API keys are now set with the HTTP Authorization request header, making your API keys more secure.

While the old way of passing API keys continues to work, after a period of time this will be permenatly removed so we urge users to update API calls accordingly.
https://www.braze.com/docs/api/api_key/#how-can-i-use-it

[codecov]

Codecov Report

Merging #20 (67ab18a) into master (0c36f95) will increase coverage by 1.26%.
The diff coverage is 100.00%.

❗ Current head 67ab18a differs from pull request most recent head 4d8e02d. Consider uploading reports for the commit 4d8e02d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   93.93%   95.20%   +1.26%     
==========================================
  Files           3        3              
  Lines         231      250      +19     
==========================================
+ Hits          217      238      +21     
+ Misses         14       12       -2     

[AnumSheraz]

The repo is of no use until this gets merged.
So when ?