Consider environment variable equivalents for some constants

[costdev]

Scenario

A host wants sites to use the host's choice of install/update source.

Current solution

The host can add default values for the existing AspireUpdate constants to wp-config.php.

Problem

Someone with file access, or even a plugin, may accidentally remove the host's constant definitions.

Impact

The site's install/update capability becomes dependent on WordPress.org, which may not be available to the host, or may not have the same packages available as the host's choice.

Additionally, this could lead to time and money spent on raising/handling support requests.

Proposed solution

Add environment variable equivalents for the following constants:

• AP_ENABLE
• AP_HOST
• AP_API_KEY
• AP_DISABLE_SSL
• AP_REMOVE_UI

Priority: Constant definitions take priority over environment variables.

This means that hosts can configure defaults which can't be accidentally removed, but can still be intentionally overridden by someone with file access, or by a plugin.