Expose createRouter from HttpService, prepare handlers for context introduction

[mshustov]

Summary

Added createRouter as a part of HttpService Setup contract.
It serves 2 purposes:

1. Forces all resources exposed by a plugin to be prefixed with plugin name. Core services may registered any urls.

  const router = createRouter();
  // handler is called when '${my-plugin-id}/path' resource is requested with `GET` method
  router.get({ path: '/path', validate: false }, (context, req, res) => res.ok({ content: 'ok' }));

2. Restricts access to plugin dependencies within handler context.

  const router = createRouter();
  
  router.get({ path: '/path', validate: false }, (context, req, res) => {
    // has access to `findObject` method of context, exposed by core service, plugin itself or plugin dependencies 
    const data = await context.findObject(request.params.id);
    return response.ok(data);
  };

Dev docs are covered by #41894

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] 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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