fix(no-misused-observables): accept Observable-returning function for union types

[JasonWeinzierl]

Allow returning Observables from union types of properties like:

interface Hook {
  onInit?: ((field: string) => void) | ((field: string) => Observable<any>);
}

const hook: Hook = {
  onInit: field => of(field),
};

This corner case was skipped when the initial rule was created based on tseslint's no-misused-promises.

Resolves #193

[github-actions]

LCOV of commit 734c01c during .github/workflows/ci.yml #616

Summary coverage rate:
  lines......: 96.7% (4495 of 4649 lines)
  functions..: 96.2% (276 of 287 functions)
  branches...: no data found

Files changed coverage rate:
                                                                    |Lines       |Functions  |Branches    
  Filename                                                          |Rate     Num|Rate    Num|Rate     Num
  ========================================================================================================
  src/rules/no-misused-observables.ts                               | 4.0%    454| 0.0%    18|    -      0