[Maps] Support custom vector tiles defined by user

[nickpeihl]

Describe the feature:
Users can define custom raster tile services for the Maps app in kibana.yml or via the UI. We should extend this to support custom vector tile services.

Describe a specific use case for the feature:
A user in an air-gapped environment may wish to display basemaps from their own internal vector tile service. The Mapbox Style Specification is likely the most common use case for defining styles and data for vector tiles. So a user may only need provide a custom URL to a JSON object that matches the specification.

[elasticmachine]

Pinging @elastic/kibana-gis

[nyurik]

We should add a layer that allows users to point it to a custom style file (it should work with out stock styles too, e.g. this one)

[thomasneirynck]

#64488 addresses this (from a data-format standpoint).

There's different standards that are built on top of this:

• TileJson support ([Maps] Support TileJson spec #70146)
• Mapbox-style spec support ([Maps] Support mapbox vector style sheets #70366)

Both are different:

• Supporting .mvt tiles and/or TileJons maps cleanly to a introducing a new single data-source in Kibana Maps. It does not impact styling.
• Supporting mapbox style-sheets maps to multiple Kibana layers. There's also a mismatch in semantics between a layer in mapbox (which only targets a single styling-property) and one in Kibana (a layer has multiple styling-properties).