Change private "fields" and "injector" to protected

[Angi-Kinas]

Description

This RP changes private "fields" and "injector" in the FieldsService to protected in order to be able to extend the class.

Quality Assurance Checklist

• Commit history is devoid of any merge commits and readable to facilitate reviews
• If new logic ⚙️ is introduced: unit tests were added
• If new user stories 🤏 are introduced: E2E tests were added
• If new UI components 🕹️ are introduced: corresponding stories in Storybook were created
• If breaking changes 🪚 are introduced: add the breaking change label
• If bugs 🐞 are fixed: add the backport <release branch> label
• The documentation website 📚 has received the love it deserves

[github-actions]

Affected libs: feature-search, feature-router, feature-map, feature-dataviz, feature-record,
Affected apps: metadata-editor, datahub, demo, webcomponents, search, map-viewer,

• 🚀 Build and deploy storybook and demo on GitHub Pages
• 📦 Build and push affected docker images

[github-actions]

📷 Screenshots are here!

[tkohr]

Thanks @Angi-Kinas !