[UI] Update extension configuration and readme instructions

[krumware]

The package.json needs to be updated to to match latest ui-plugin spec, and to target the 2.7 and 2.8 Rancher Management Server release ranges. This will protect extension users from hitting potentially breaking changes during Rancher 2.9 updates, and define the appropriate dependencies.

In addition, the instructions for installing the extension via the Rancher UI is currently not documented. It should also be clarified that the extension is in Alpha until formal release.

Reelated to: #192