feat(aws-datastore): Make the reachability component configurable

[manueliglesias]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
N/A

Description of changes:
Make the reachability component optionally configurable by consumers of DataStore. This will open up the ability to define what "online" means if the default behavior is not enough (e.g. the default reachability monitor for the platform may report online=true based solely on the status of the radio but not the actual internet connectivity), e.g. a consumer may want to define the online status as "my app can reach X endpoint".

The following snippets show a fake monitor that is "manually" toggleable:

Amplify.addPlugin(AWSDataStorePlugin.builder()
      .reachabilityMonitor(ToggleableReachability.getInstance())
      .build()

public class ToggleableReachability implements ReachabilityMonitor {
    private static ToggleableReachability instance = new ToggleableReachability();
    private boolean isOnline;
    private final Subject<Boolean> subject;
    private boolean isConfigured;

    public static ToggleableReachability getInstance() {
        return instance;
    }

    private ToggleableReachability() {
        this.isOnline = true;
        this.subject = BehaviorSubject.createDefault(isOnline);
    }

    @Override
    public void configure(@NonNull Context context) {
        configure(context, null);
    }

    @Override
    public void configure(@NonNull Context context, @NonNull ConnectivityProvider connectivityProvider) {
        this.isConfigured = true;
        subject.onNext(isOnline);
    }

    @NonNull
    @Override
    public Observable<Boolean> getObservable() {
        if (!isConfigured) {
            return Observable.error(new DataStoreException(
                    "ReachabilityMonitor has not been configured.",
                    "Call ReachabilityMonitor.configure() before calling ReachabilityMonitor.getObservable()"
            ));
        }

        return subject;
    }

    public void toggle() {
        isOnline = !isOnline;
        subject.onNext(isOnline);
    }
}

How did you test these changes?
Implemented a custom ReachabilityMonitor as shown above, wired it to a UI component, toggled it and verified that DataStore was correctly toggling between states.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #2307 (acc1651) into main (48e294b) will decrease coverage by 0.01%.
The diff coverage is n/a.

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