Allow printing by freely setting the scale factor

[tdipisa]

Description

The request is to enhance the current MapStore print to allow printing maps by freely setting the scale factor from the preview panel, entering the desired value.

It should be possible to set in config.yml the configuration disableScaleLocking: true. This allows the client to do requests with any scale value. The scale values have to be anyway set in config.yml, but with this setting, a request with a different scale will not fail.

We should:

• Enable fractional zooms with OL so that it is possible for users to insert manually the resolution and see it reflecting on the map
• Make the scale list tool editable to allow to insert the desired scale. This should be enabled by plugin config with a new property that implies that the disableScaleLocking is set to false on the config.yml side (a proper documentation shoudl be indeed included in the JS doc of the plugin)

What kind of improvement you want to add? (check one with "x", remove the others)

• Minor changes to existing features
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

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