Support async ErrorStateMatcher

#### Please describe the feature you would like to request.

Currently, `ErrorStateMatcher` doesn't support return type `Promise` and `Observable`. Its return type is only `boolean`.

#### What is the use-case or motivation for this proposal?

I need to show `` when checked `input` is invalid after validation. This validation does need to call HTTP request(async). For example, when ID field in registration form is not unique, i should show `` because registration form doesn't allow to use the input's value as duplicated ID with other user. To provide this feature, `ErrorStateMatcher` has to support return type as `Promise` or `Observable`.

Start by reading the ErrorStateMatcher API and the existing mat-error validation flow, focusing on how the input's validity is checked. Compare the requested Promise and Observable cases, then define completion as async validation results reliably causing the appropriate mat-error state for duplicate IDs.

Written by the indexing model from the issue text.