[Maps] Proxied File Manifest can return incorrect format

[nickpeihl]

Kibana version: 7.8.0, master

Describe the bug:
When multiple formats are available in the Files manifest, the maps proxy rewrites the layer format object to only include the first item in the formats array. However, this item may not necessarily be the preferred default format. This can cause an error like features.featureCollection not found when a layer supports multiple formats.

Steps to reproduce:

1. Enable map.proxyElasticMapsServiceInMaps: true in kibana.yml
2. Add map.emsFileApiUrl: https://storage.googleapis.com/elastic-ems-dev-feature-layers-vector to kibana.yml
3. Try to add the "Administrative Regions" layer to a new Map.

Expected behavior:
The proxied manifest should either include all available formats or a create a single item array with the default format rather than assume the first item in the array is the default.

[elasticmachine]

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