Fix(external-api): Support blur and none types in setVirtualBackground.

[san-rizz-777]

Problem

setVirtualBackground was hardcoded to VIRTUAL_BACKGROUND_TYPE.IMAGE
making it impossible to set blur or remove background via external API.

Changes

modules/API/API.js

• Accept backgroundType parameter instead of hardcoding IMAGE
• Support 'blur', 'image', 'desktop-blur', 'none'

modules/API/external/external_api.js

• Update setVirtualBackground signature to pass backgroundType through

Usage

// Blur background
api.setVirtualBackground(true, 'blur');

// Custom image
api.setVirtualBackground(true, 'image', 'https://example.com/bg.jpg');

// Remove background
api.setVirtualBackground(false, 'none');

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.