[7.x] Add x-pack plugin for new platform server licensing information

[mshustov]

• Add x-pack plugin for new platform server licensing information

• Update x-pack translations

• Implement core plugin interface for licensing plugin:

• Rename references to service to plugin

• Use CoreStart in licensing start method

• Fix outstanding reference error

• Fix type check errors

• Address outstanding review comments

• Fix type error in tests

• Address review comments, move polling logic to standalone

• Split up test files

• Fix bad reference in test

• Use relative reference to poller util

• Add more plugin tests to address review comments

• Fix different manners of config generation in licensing plugin

• Update test fixtures

• Fix path to test fixtures

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Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

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