[FluentButton] Update the Button custom style for BackgroundColor and Color properties

[dvoituron]

[FluentButton] Update the custom Button style for the BackgroundColor and Color properties.

1. Remove the custom-id attribute to use the default id attribute (if not set, a default value is assigned).
2. Use the `InlineStyleBuilder' utility to create an inline custom style like this.

<FluentButton BackgroundColor="#ff0000" Color="#00ff00">My button</FluentButton>

will generate...

<style>
  #f7e40a894::part(control) { background: padding-box linear-gradient(#ff0000, #ff0000), border-box #ff0000; color: #00ff00; }
  #f7e40a894::part(control):hover { opacity: 0.8; }
</style>

<fluent-button type="button" id="f7e40a894">My button</fluent-button>

[vnbaaij]

Nice!