[Enterprise Search] Add helpers for flash messages

[scottybollinger]

Summary

With the current FlashMessages implementation, the setting of a simple message is quite verbose. This PR exposes simple methods that set single-string messages, which covers most use cases in the logic files.

Before

FlashMessagesLogic.actions.setFlashMessages({
  type: 'success',
  message: 'Successfully created bird.',
});

After

setSuccessMessage('Successfully created bird.');

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser

[cee-chen]

Love it!

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: c047767

Build metrics

@kbn/optimizer bundle module count

id	value	diff	baseline
enterpriseSearch	272	+1	271

async chunks size

id	value	diff	baseline
enterpriseSearch	433.7KB	+956.0B	432.7KB

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@elasticmachine merge upstream