[SDK-2118] Added a method to set API URL

[nsingh-branch]

Reference

SDK-2118 -- Added a method to set API URL

Summary

Added a new method to the Branch class for setting the API base URL to a custom value. This also includes adding a check to the getBaseURL method to check for a custom URL.

Motivation

To allow clients to provide custom API URLs and gain parity with our Android SDK.

Type Of Change

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

Testing Instructions

Call the setAPIURL method in the AppDelegate and observe the change.

Branch *branch = [Branch getInstance];
[branch setAPIUrl:@"https://api2.branch.io"];

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

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (bf6e9ee) 50.73% compared to head (9a0722e) 50.60%.
Report is 3 commits behind head on master.

❗ Current head 9a0722e differs from pull request most recent head 19692a0. Consider uploading reports for the commit 19692a0 to get more accurate results

Files	Patch %	Lines
BranchSDK/Branch.m	0.00%	7 Missing ⚠️
BranchSDK/BNCServerAPI.m	42.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1350      +/-   ##
==========================================
- Coverage   50.73%   50.60%   -0.14%     
==========================================
  Files          68       68              
  Lines       10081    10070      -11     
  Branches     3864     3862       -2     
==========================================
- Hits         5115     5096      -19     
- Misses       4708     4714       +6     
- Partials      258      260       +2     

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