Implement stats Rest and Realtime clients

[ben-xD]

Implement following methods in Rest and Realtime

ably-flutter/lib/src/platform/src/realtime/realtime.dart

Lines 104 to 105 in b3cb8bf

	@override
	Future<DateTime> time() {

and

ably-flutter/lib/src/platform/src/rest/rest.dart

Line 61 in b3cb8bf

@override

These methods throw UnimplementedError();.

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