[Image] Support for fetchpriority attribute

## Feature Request

In order to make AEM pages more performant, it would be good that the Image component supports an authorable _fetchpriority_ attribute.

**Describe the solution you'd like**

A select field in the Design Dialog and Edit Dialog to set the _fetchpriority_ attribute as desired (_high_, _low_, _auto_).

**Documentation**

A select field in the Design Dialog would configure the default value at policy level.
A select field in the Edit Dialog would allow to configure the value at component level, for example to optimize the Largest Contentful Paint metric by prioritizing a hero image.

Locate the Image component's Design Dialog, Edit Dialog, and rendering entry points; start by tracing how policy-level and component-level settings are represented. Confirm that high, low, and auto can be selected at both scopes and that the chosen value appears as the fetchpriority attribute in the rendered image.

Written by the indexing model from the issue text.