[ts][pixi-v8][pixi-v7] Support for Base64-Encoded Assets

[davidetan]

The spine-pixi runtime does not directly support base64-encoded assets.

Currently, to make Spine objects work with base64 assets, we need to replicate the functionality of our custom atlas and skeleton loaders.

This forum post demonstrates the current workaround. As shown, the process is not straightforward, requiring users to manually add assets to the Pixi cache, and to manually loop over all atlas pages and assign the corresponding PixiSpine textures.

This issue proposes enhancing our custom loaders to support base64-encoded assets natively, streamlining the process for users.