[Feature Anywhere] Add a vis_layers expression function type

[ohltyler]

As part of #2880, we propose to have a standardized way for plugins to format their overlay data into a VisLayer that can be processed by the downstream visualizations for rendering this additional data. For more information on the VisLayer overlay data model, see #2882.

To accomplish this, we can introduce a vis_layers expression function type, and allow plugins to register their own functions of this type. During the visualization rendering process, all relevant expression functions can be fetched via saved objects (see #2893 for details on the saved objects design), and built into a single pipeline that can be ran to compile an array of VisLayers. The visualization can then process these VisLayers accordingly, and render the additional data.

The I/O of this new expression function will be the same - an array of VisLayers. The purpose of the function itself will be to take in the array, construct a VisLayer of their own using their own data, append it to the array, and pass it on as output. This design allows for an arbitrary number of these function types to be executed in an arbitrary order, allowing any plugin to add any number of VisLayers to a particular visualization.

We can illustrate how this may work with a simple example:
[] -> plugin-1-expr-fn -> [vis-layer-1] -> plugin-2-expr-fn -> [vis-layer-1, vis-layer-2]

Open questions:

1. Similar to [Feature Anywhere] Add a augment-vis saved object type #2893, where should this function be maintained & registered inside of OpenSearch Dashboards? My preferred solution is that this is maintained in a standalone feature-anywhere plugin

[ohltyler]

Closing via #3108