Add scale method option for layout scale bars (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request qgis/QGIS#58904
Author: @nyalldawson
QGIS version: 3.40

**Add scale method option for layout scale bars **

PR Description:

Instead of always calculating the scale along the bottom of the
map, expose a choice of methods to the user (along bottom,
middle, top, or average of the three measurements)

For new scalebars, default to the average method, which better
handles the scenario where the scale at the top or bottom of
the map cannot be calculated (eg when the top/bottom of the map
falls just outside valid areas for the map's crs)

This fixes one of the most common scenarios which cause scale
bar widths to blow out to massive sizes

Refs qgis/QGIS#55240

Commits tagged with [need-docs] or [FEATURE]