Ordering the enhancers by group name for OpenAPI spec enhancer service

Suggestion

See discussion in https://github.com/strongloop/loopback-next/pull/4258#discussion_r355542472, when load spec enhancers, people should be able to specify order for them.

A proposal would be using a combination of group and alphabet order like how we load observers group by group in https://loopback.io/doc/en/lb4/Life-cycle.html#observer-groups.

Use Cases

When define an enhancer, you can specify the tag in the binding template like @bind(asSpecEnhancer({CoreTags.OAS_SPEC_ENHANCER_GROUP: 'path'}))

Then specify the enhancer order like:

app
  .bind(CoreBindings.OAS_SPEC_ENHANCER_OPTIONS)
  .to({orderedGroups: ['path', 'component', 'info']});

please note this is just a proposal, the story owner can think of better design to provide the group name and specify the group order through options

For enhancers in the same group, they are loaded according to the name(alphabetically)

Examples

Define a path spec enhancers with tag path and a component enhancer with tag component

@bind(asSpecEnhancer({CoreTags.OAS_SPEC_ENHANCER_GROUP: 'path'}))
export class PathSpecEnhancer implements OASEnhancer {
  name = 'path';
  modifySpec(spec: OpenApiSpec): OpenApiSpec {
    const PathPatchSpec = {
      // some spec
    };
    const mergedSpec = mergeOpenAPISpec(spec, PathPatchSpec);
    return mergedSpec;
  }
}

@bind(asSpecEnhancer({CoreTags.OAS_SPEC_ENHANCER_GROUP: 'component'}))
export class ComponentSpecEnhancer implements OASEnhancer {
  name = 'component';
  modifySpec(spec: OpenApiSpec): OpenApiSpec {
    const ComponentPatchSpec = {
      // some spec
    };
    const mergedSpec = mergeOpenAPISpec(spec, ComponentPatchSpec);
    return mergedSpec;
  }
}

Then specify the enhancer order like:

app
  .bind(CoreBindings.OAS_SPEC_ENHANCER_OPTIONS)
  .to({orderedGroups: ['path', 'component', 'info']});

Acceptance criteria

• Allow the OAI spec enhancer service to load enhancers by group names.

Start by locating the OAI spec enhancer service and the CoreBindings.OAS_SPEC_ENHANCER_OPTIONS binding; review the linked pull-request discussion and observer-group loading behavior. Done means the service loads enhancers in configured group order and alphabetically within each group, with tests covering the behavior.