Asynchronously returns a symbol based on the provided list of keys. This function is useful for obtaining a single, multi-layer symbol from supplied keys. This is commonly used to create symbols and graphics on-the-fly. For example, assume XYZ style has a symbol with key "abc" which is the central symbol and another symbol with key "pqr", which could be the modifier/echelon placed at some offset from central geometry. So if you provide these two keys as attributes then symbol style will find and assemble a symbol accordingly. Once you have that symbol, you can apply it to a Graphic or Renderer, obtain its swatch image, or serialize to JSON. If the Symbol style is not loaded then starting this task will start the load cycle.