Make custom inspector adapters registerable from the visType

[timroes]

This is an issue created from a discussion @ppisljar and I had.

Originally you could register a custom inspector adapter via the vis type, by adding a customAdapters flag to it. This has been removed over some of the refactorings. While building the Vega inspector panel we'd need this capabilities back now, since we need to register a specific adapter just for Vega. There is currently no proper other place where you could register it, that it would work with all cases, e.g. the PR currently registering it from within the expression function, which means, that it's not available before rendering, thus makes the embeddable not properly able to detect if the adapter is there. (Also it's mutating some context passed into the expression function, which we def shouldn't do.)

So this issue tracks adding back the capability of adding custom inspector adapters via the vis type. More notes from the chat:

visualize embeddable creates executor instance, it has the ability to provide inspectorAdapters to it, but it doesn't do so and just expects the default set (data, request)
so we need to add that back it seems, visType needs to list what adapters to initialize
and visualize embeddable needs to use that information to pass the initial list of adapters to executor

[alexwizp]

Will be fixed in scope of #70941