[Maps] add migration to switch tile layers with EMS_TMS source to vector tile layer

[nreese]

fixes #43488

This PR adds a migration script to migrate map saved objects, converting tile layers with EMS_TMS source to vector tile layers

To test,
Import the following saved object before checking out the branch

{"attributes":{"bounds":{"coordinates":[[[-180,84.21821],[-180,-84.21821],[180,-84.21821],[180,84.21821],[-180,84.21821]]],"type":"Polygon"},"description":"","layerListJSON":"[{\"sourceDescriptor\":{\"type\":\"EMS_TMS\",\"isAutoSelect\":true},\"id\":\"9ce07a33-68ad-4207-ad5a-c0995558b40b\",\"label\":null,\"minZoom\":0,\"maxZoom\":24,\"alpha\":1,\"visible\":true,\"applyGlobalQuery\":true,\"style\":{},\"type\":\"TILE\"}]","mapStateJSON":"{\"zoom\":0.5,\"center\":{\"lon\":0,\"lat\":0},\"timeFilters\":{\"from\":\"now-15m\",\"to\":\"now\"},\"refreshConfig\":{\"isPaused\":false,\"interval\":0},\"query\":{\"query\":\"\",\"language\":\"kuery\"},\"filters\":[]}","title":"map with raster EMS tiles","uiStateJSON":"{\"isLayerTOCOpen\":true,\"openTOCDetails\":[]}"},"id":"e1e6b040-c45f-11e9-b475-b74e327fb341","migrationVersion":{"map":"7.2.0"},"references":[],"type":"map","updated_at":"2019-08-21T22:06:10.244Z","version":"Wzc0LDFd"}

Open the saved object in the maps application and verify the map is loading png tiles by viewing the network tab in the browser dev tools.

Stop Kibana, checkout this branch, then restart kibana. Re-open this saved object in the maps app. Verify tiles are now vector tiles (.pbf, Protocolbuffer Binary Format) in network tab

[elasticmachine]

Pinging @elastic/kibana-gis

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[nickpeihl]

lgtm!