chore(smithy): Improve union DX

[dnys1]

Currently to construct a union member, you need to pass an instance of the represented object, for example:

sealed class AnalyticsFilter extends SmithyUnion<AnalyticsFilter> {
  const factory AnalyticsFilter.tag(Tag tag) = AnalyticsFilterTag;
}

void usage() {
  final filter = AnalyticsFilter.tag(Tag(key: key, value: value));
}

This was uncovered as a less-than-desirable DX when exploring the config object. Instead, we can inline the parameters needed to construct the object to save the extra construction:

sealed class AnalyticsFilter extends SmithyUnion<AnalyticsFilter> {
  factory AnalyticsFilter.tag({
    required String key,
    required String value,
  }) =>
      AnalyticsFilterTag(_i2.Tag(
        key: key,
        value: value,
      ));
}

void usage() {
  final filter = AnalyticsFilter.tag(key: key, value: value);
}

This also means you don't need to know the underlying object type which is often just a required abstraction for Smithy - more often than not they are just anonymous groups of properties.

The old behavior is still available by invoking the union subclass constructor directly, e.g.

void usage() {
  final filter = AnalyticsFilterTag(Tag(key: key, value: value));
}

The base branch was changed.