[gis_web] Improves renderButton API exports.

[ditman]

The previous version of renderButton required exposing DomHtmlElement from this library to the world. We don't want that, instead, the JS-Interop API should accept anything that is an HtmlElement (for now: Object).

This PR also exposes the classes needed to configure the button that is rendered by the method.

Adds a small integration test to check that the behavior is valid.

(This PR removes the dependency on, and disallows the usage of the jwt_decoder package.)

Pre-requirement for: flutter/flutter#121781

Pre-launch Checklist

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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[stuartmorgan]

LGTM