[Feature Request] Provide ability to load vector tiles from a local function

Nobody has claimed this yet.

## Motivation

I've seen this mentioned a few times, but have not seen an actual request or feature for it. So here it is!

* Client-side generated tiles
* Client-side generated grids
* Anything where the vector tile generation can be offloaded to the client on a tile-by-tile basis
* Alternative caching mechanism or logic that needs to be maintained outside of Mapbox
* Providing an alternative to the delay/lag of loading large datasets that may be much larger than the current viewport, instead it could be generated & rendered tile-by-tile client-side
* Similarly, a changing dataset could be more performant if more use-case specific tile-generating & caching strategies could be employed, and the vector tiles results just fed to mapbox

**Similar Issues:**

* https://github.com/mapbox/mapbox-gl-js/issues/9897

## Design Alternatives

Unknown

## Design

Prove the ability to pass a function as a vector tile source for a layer. Instead of just a URL.

For instance a function `getTile(x,y,z)` could return the appropriate map tiles for mapbox to use for some layer.

### Mock-Up

Unknown

### Concepts

Unknown

### Implementation

Unknown

Start by reviewing the existing vector tile source API and the related issue #9897. Determine how a source function such as getTile(x,y,z) would provide tiles to a layer, including the expected return behavior. Done means a documented, tested API can accept a local function and render its generated vector tiles.

Written by the indexing model from the issue text.