Use of array_key_first breaks compatibility with php < 7.3

[nattyg93]

Describe the bug https://github.com/MinnPost/object-sync-for-salesforce/blob/c0d101fe5e333c42160f1ac5cc313446f7dd68f3/classes/class-object-sync-sf-salesforce.php#L408

The usage of array_key_first above causes the plugin to be broken with php < 7.3.

To Reproduce Steps to reproduce the behavior:

1. Install object-sync-for-salesforce 2.0.0
2. Use php < 7.3 (e.g. 7.0)
3. Try to access the admin screen
4. Error due to function not existing

Expected behavior The readme indicates that this plugin is compatible with php 5.6, but does not run with php 7.0

Environment (please complete the following information):

• WordPress Version: 5.6
• PHP Version 7.0

I worked around the issue by adding the following:

if (!function_exists('array_key_first')) {
    function array_key_first(array $arr) {
        foreach($arr as $key => $unused) {
            return $key;
        }
        return NULL;
    }
}

[jonathanstegall]

Thanks. I'm happy to add a fix for this. Some things to note:

1. Since this code is in a class, I've added a different fix in pull request #411. I wonder if you'd be willing to make sure this works as you'd expect.
2. I've noticed that adding a global polyfill for this to WordPress has been assigned for version 5.9. Hopefully this will occur and increase the efficiency for users who need that function.

[nattyg93]

Thanks for the quick turn around on this - I've tested the fix and it solves the issue.