Mattermost-server: Override LHS bot icon in demo plugin

[ali-farooq0]

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Notes: Jira ticket

We've extended PluginAPI as part of

It would be great for the demo plugin to use this functionality.

I'll be attaching a few SVG icons that can be used for the demo plugin

[QamarFarooq]

Ill take this one guys

[QamarFarooq]

pr created https://github.com/mattermost/mattermost-plugin-demo/pull/59