Expose a REST end-point through standard routing

Overview

Provide a binding for CRM_Utils_REST in the conventional routing system.

Example use-case

1. Create a D8 site
2. Create user/contact and set their API key
3. Access the rest end-point with their API key

Current behavior

• There is a binding through the extern/rest.php.
• There are tests for the REST contract in E2E_Extern_RestTest
• The end-point is difficult to use/deploy in environments like D8.

Proposed behavior

• There is another binding civicrm/api/rest which speaks the same protocol.
• The RestTest hits both end-points (depending on what's valid/available for the given build).
• References to the REST endpoint (eg RestTest and API Explorer) are constructed via CRM_Utils_System::externUrl('extern/rest')

Comments

• This is an offshoot of drupal#7 (closed) and cloud-native#16 (moved)
• This sounds easier than it is.
• There's a draft/WIP: https://github.com/civicrm/civicrm-core/pull/17952
• The protocol's authentication-handler is tied into CRM_Utils_System::loadBootstrap(). This needs refactoring/reworking to apply to be valid in a standard route.