[Maps] Support term-joins for vector tile format

[thomasneirynck]

Describe the feature:

When the .mvt scaling type is used, term joins should be supported

Describe a specific use case for the feature:

Term-joins are a client-side operation in Maps. By introducing .mvt tiles, data-generation was pushed to the server. Term-joins cannot be supported in the exact same way because of this.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[thomasneirynck]

let's close for now. No direct ask.