[MM-40214] Implement App Bar icon

[mickmister]

Summary

This PR makes it so the plugin registers its own component to the App Bar. The core webapp makes it so the component is only mounted if the AppBarEnabled feature flag is on. This logic is only in the core webapp, and not in the plugin's source.

The plugin registers its usual channel header component, while simultaneously register the App Bar component. This is so the webapp can show one or the other when the App Bar feature flag is enabled or disabled. The plugin itself then doesn't need to watch this feature flag, and the webapp can manage the effects of the flag.

Ticket Link

Fixes https://mattermost.atlassian.net/browse/MM-40214

[dipak-demansol]

LGTM, Tested Same build with Two server where 1st server had app-bar enabled and on the 2nd server app-bar was disabled.