Allow defining a custom config spec with WP_CLI_CONFIG_SPEC_PATH constant

Fixes #5649

Proposed changes

Allows use of a custom config-spec.php file (for registering global parameters or disabling existing) by defining a WP_CLI_CONFIG_SPEC_PATH constant.

The use of a constant (instead of an environment variable) enables the platform operator to register a platform-specific config-spec.php, and prevent later changes to it.

Testing instructions

To come...