PWA-3285 : Update networkUtils.js to check saveData mode

[glo85315]

Description

Add data saver verification before prefetching images.

Data Savers allow users to signal to a site if they would prefer lighter data-usage where possible. PWA Studio respects Save-Data preferences where navigator.connection.saveData is supported by the browser.

Related Issue

Closes #3708

Acceptance

Verification Stakeholders

Specification

Verification Steps

Test scenario(s) for direct fix/feature

Test scenario(s) for any existing impacted features/areas

Test scenario(s) for any Magento Backend Supported Configurations

Is Browser/Device testing needed?

Any ad-hoc/edge case scenarios that need to be considered?

Screenshots / Screen Captures (if appropriate)

Breaking Changes (if any)

Checklist

• I have added tests to cover my changes, if necessary.
• I have added translations for new strings, if necessary.
• I have updated the documentation accordingly, if necessary.

[pwa-studio-bot]

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