[Telemetry] Centralized opt in service

[Bamieh]

Summary

• Centralizes Telemetry Opt-in service (used to be in 3 different places (home/license, and telemetry).
• Moves all vars/configs logic to server.
• Fixes banner poping up before welcome screen appears.
• Gets rid of $injector and angular Provider in telemetry to further ease moving to NP.
• Fixes welcome card example overlay z-index.
• Gets rid of duplicate fetchTelemetry code.
• Converts all telemetry plugin code to Typescript 🎉 .
• Fully migrates public folder to NP

Closes #45428

Remaining:

• Fix tests
• Add tests for the new opt-in service
• Fix kibana when kiaban config telemetry.enabled is set to false

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: ad80a6a

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: 9900f66

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: c8272f2

[snide]

Sass changes are minimal. Didn't look beyond the code.

[Bamieh]

@elasticmachine merge upstream

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: 1d61a74

[joshdover]

@Bamieh What's the status here? My main concern is getting those uiExport hacks out of legacy and into the New Platform.

[Bamieh]

@joshdover This has been pushed back due to the 7.5.0 release urgent features.

With all the changes introduced to the telemetry repo in the meantime; I believe i'll be closing this PR and cherrypicking files from the branch. I have two PRs migrating things into NP for this week (cloud and ui_metrics), I'll revisit migrating the hacks afterwards.