Add help popover with pipeline type info

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What does this MR do and why?

These changes add to the widget pipeline a helper popover component containing info regarding the type of pipeline. The popover will be only shown if matches one of the types. This prevents showing empty info when there is inconsistency with the data received.

To ensure the event_type_name matches the correct title and content, I created a new constants.js file to store the necessary values that align with the backend data.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit an MR such as http://127.0.0.1:300/toolbox/gitlab-smoke-tests/-/merge_requests/2
2. You should observe a help icon in the pipeline widget
3. Open the component mr_widget_pipeline.vue and change the computed prop pipelineTypeHelpPopoverOptions variable eventTypeName to one of the values below, and repeat 1 and 2.

export const PIPELINE_EVENT_TYPE_MERGE_TRAIN = 'Merge train pipeline';
export const PIPELINE_EVENT_TYPE_MERGED_RESULT = 'Merged result pipeline';
export const PIPELINE_EVENT_TYPE_MERGE_REQUEST = 'Merge request pipeline';

Related to #13631 (closed)