[SDK-2328] Added API URL check for Branch.json

[nsingh-branch]

Reference

SDK-2328 -- Support setting Branch API URL in Branch.json

Summary

Added a new field to branch.json for setting the Branch API URL. The config check and setAPIUrl call are done in Branch.m's initWithInterface() in the same place as the deferInitForPluginRuntime() and enableLogging() checks.

Motivation

To support setting the API URL in our plugins like React Native.

Type Of Change

• [x] New feature (non-breaking change which adds functionality)

Testing Instructions

Add "apiUrl": "https://api.branch.io" to a branch.json file and observe the API URL properly change for each request.

cc @BranchMetrics/saas-sdk-devs for visibility.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 51.17%. Comparing base (1b03a81) to head (84ef9b2). Report is 12 commits behind head on master.

Files	Patch %	Lines
Sources/BranchSDK/Branch.m	33.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1371      +/-   ##
==========================================
- Coverage   51.27%   51.17%   -0.10%     
==========================================
  Files          66       66              
  Lines       10092    10098       +6     
  Branches     3697     3703       +6     
==========================================
- Hits         5175     5168       -7     
- Misses       4654     4661       +7     
- Partials      263      269       +6     

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