fix(no-misused-observables): false positive for JSX attributes

[JasonWeinzierl]

Missed corner case from the original implementation of this rule when imitating no-misused-promises.

We didn't add unit tests for the non-void case, so this was triggering the rule:

        import { Observable, of } from "rxjs";

        interface Props {
          foo: () => Observable<number>;
        }
        declare function Component(props: Props): any;

        const _ = <Component foo={() => of(42)} />;

essentially blocking any prop from ever being an Observable-returning function. We need to check if the contextual type of the prop is void-returning and if the expression returns an Observable before reporting a violation.

[github-actions]

LCOV of commit 12b9a93 during .github/workflows/ci.yml #609

Summary coverage rate:
  lines......: 96.7% (4485 of 4639 lines)
  functions..: 96.2% (275 of 286 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%    449| 0.0%    18|    -      0