refactor(Legend): abstract chart legend props

What does this MR do?

Abstracts legend props since they are repeated through multiple files

Possible approach

We could abstract these to a utility file that lives alongside the charts and then each component can just import them by default:

// ../../../utils/charts/utils

export const  legendAverageText = {
    type: String,
    required: false,
    default: LEGEND_AVERAGE_TEXT,
  };

export const legendMaxText=  {
    type: String,
    required: false,
    default: LEGEND_MAX_TEXT,
  };

export const legendMinText = {
    type: String,
    required: false,
    default: LEGEND_MIN_TEXT,
  };

export const legendCurrentText = {
    type: String,
    required: false,
    default: LEGEND_CURRENT_TEXT,
  };

export const legendLayout = {
    type: String,
    required: false,
    default: LEGEND_LAYOUT_INLINE,
    validator(layout) {
      return [LEGEND_LAYOUT_INLINE, LEGEND_LAYOUT_TABLE].indexOf(layout) !== -1;
    },
  };


// area.vue / line.vue / stacked_column.vue

import { legendAverageText, legendCurrentText, legendLayout, legendMaxText, legendMinText } from '../../../utils/charts/utils'

formatTooltipText: {
      type: Function,
      required: false,
      default: null,
    },
    legendMaxText,
    legendMinText,
    legendLayout,
    legendCurrentText,
    legendAverageText

This really helps cut down on the repeated code and lets us adhere to the open closed principal of the props being open to extension but closed for modification, which I think would be useful for the charts legend.

Does this MR meet the acceptance criteria?

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