feat(SDK-2237): add setAPIUrl getAPIUrl public methods

add setAPIUrl getAPIUrl public methods, unit tests

setAPIUrl : Sets a custom base URL for all calls to the Branch API

getAPIUrl : returns the base URL for all calls to the Branch API

usage :

branch.setAPIUrl("https://api.branch.io");

branch.getAPIUrl();

Fixes # (issue) : SDK-2237

Type of change : feature

Please delete options that are not relevant.

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

[ x] Unit test
[ ] Integration test

JS Budget Check

Please mention the size in kb before abd after this PR

Files	Before	After
dist/build.js.
dist/build.min.js

Checklist:

[ ] My code follows the style guidelines of this project
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[ ] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] New and existing unit tests pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules
[ ] I have checked my code and corrected any misspellings

Mentions:

List the person or team responsible for reviewing proposed changes.

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

BranchMetrics / web-branch-deep-linking-attribution