Allow out-of-box token based authentication in API Explorer

[dhmlau]

Suggestion

We provided some documentation on how to enable the "Authorize" button in API Explorer in the example-shopping application so that token based authentication is possible inside the API Explorer. https://loopback.io/doc/en/lb4/Authentication-Tutorial.html#specifying-the-security-settings-in-the-openapi-specification

It would be great to have out-of-box support. The initial work is done through #4258. The next step is to complete the whole user scenario:

• Add OpenAPI enhancer service in @loopback/rest Add OpenAPI enhancer service in @loopback/rest #4380
• Add bearer auth scheme as the default security scheme Add bearer auth scheme as the default security scheme #4386
• (nice to have) Ordering the enhancers by group name for OpenAPI spec enhancer service Ordering the enhancers by group name for OpenAPI spec enhancer service #4385

cc @jannyHou

[dhmlau]

Closing as done.