Add filter for extension author credit link

[johnalarcon]

Description

Add a filter that allows extension authors to append their own credit link to the view(s) created by their extension(s).

Context

The desire is to keep the interface consistent.

[johnalarcon]

This has been implemented and can be used as follows in the next release:

add_filter('codepotent_update_manager_extension_footer_{your-slug-name}', 'some_function_name');
function some_function_name($text) {
    return 'Featuring <a href="#">My Cool Extension</a> by <a href="#">Author Name</a>';
}

Note 1: all HTML is stripped except for the <a> tag which can use both the href and title attributes.

Note 2: if your extension creates multiple views, a filter should be used for each distinct slug.

[johnalarcon]

Implemented on the milestone-2.0.0 branch.