Add button for "stage" asset selector

[mfrisbey]

This PR adds another button to the sidekick for a "stage" asset selector. This is a temporary solution so that we can more safely iterate on the selector.

This button will point to the selector on the rail branch so that we can commit to that branch and see selector changes without needing to commit to main.

The button also uses an application-level client ID, and points to an AEM environment in cloud manager stage.

Test URLs:

• Before: https://main--aem-assets-boilerplate--hlxsites.hlx.page/
• After: https://selector-stage--aem-assets-boilerplate--hlxsites.hlx.page/

[aem-code-sync]

Hello, I'm Franklin Bot and I will run some test suites that validate the page speed.
In case there are problems, just click the checkbox below to rerun the respective action.

• Re-run PSI Checks

[aem-code-sync]

Page	Scores	Audits	Google
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[tmathern]

We just did something similar with Brendan!

[mfrisbey]

Closing this PR because the sidekick project advanced options has a checkbox that will force the sidekick to read its config.json properties from local.