[typescript-angular] Support rxjs 6

Description

The import of Observable in rxjs 6 is changed from previous versions. The typescript-angular generator should support the new version

Swagger-codegen version

3.0.2

Suggest a fix/enhancement

Taking any definition, the created services imports rxjs in this way:

import { Observable }                                        from 'rxjs/Observable';

With rxjs 6, the correct way to import Observable is:

import { Observable }                                        from 'rxjs';

This should be supported with a specific configuration flag, or when ngVersion is greater than 6.

The issue points to generated TypeScript Angular services and their Observable imports; inspect the typescript-angular generator templates and configuration handling first. Generate a client with RxJS 6 and ngVersion 6, then verify imports use rxjs while preserving the expected behavior for older versions.