feat: Add svg icon for all extensible areas

[GuiLeme]

What does this PR do?

Adds custom SVG icon support across all plugin extensible areas. Every icon (and iconRight, buttonIcon) property now accepts PluginIconType. To guarantee backwards compatibility the definition of this type is:

export type PluginIconType = string | {
  svgContent: <svg>; // or
  iconName: string;
};

This is just pseudo-code, check the file src/extensible-areas/common/icon/types.ts for details.

Closes Issue(s)

Closes #204

Motivation

Plugin authors were limited to BBB's built-in icon font. This change allows any custom SVG to be used as an icon in any extensible area.

More

Extensible areas updated: actions-bar-item, nav-bar-item, action-button-dropdown-item, audio-settings-dropdown-item, camera-settings-dropdown-item, options-dropdown-item, presentation-dropdown-item, screenshare-helper-item, user-camera-dropdown-item, user-camera-helper-item, user-list-dropdown-item, user-list-item-additional-information, generic-content-item.

Result:

Sample: generic-content-sidekick plugin using a custom SVG as buttonIcon

[GuiLeme]

Closing PR on behalf of #255