Plugin E2E: Make it easier to navigate to existing resources

[sunker]

What this PR does / why we need it:

This PR added a gotoAppConfigPage fixture which enabled quick navigation to an already existing app config page. This adds a new pattern on how to navigate to already existing resource:

test('test', async ({ gotoPanelEditPage }) => {
  const panelEditPage = await gotoPanelEditPage(dashboard);
})

The pattern before this was:

const page = new const panelEditPage = new PanelEditPage(
    { request, page, selectors, grafanaVersion, testInfo },
    {
      id: '5',
      dashboard: { uid: 'xyz' },
    }
  );
await panelEditPage.goto();

This PR adds corresponding fixtures for DashboardPage, PanelEditPage, AnnotationEditPage, VariableEditPage and DataSourceConfigPage.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @grafana/plugin-e2e@0.21.0-canary.812.28681a8.0
# or 
yarn add @grafana/plugin-e2e@0.21.0-canary.812.28681a8.0

[github-actions]

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[mckn]

LGTM!

[grafana-plugins-platform-bot]

🚀 PR was released in @grafana/plugin-e2e@0.21.0 🚀