[xdl] Get xdl-manifests from apiv2 endpoint

[jkhales]

How

There are two locations xdl-manifests are fetched. For the first one in packages/xdl/gulp/build-tasks.js replaced the url. Confirmed this worked by repopulating the schema cache (git diff is empty):

cd ~/expo-cli/packages/xdl
rm caches/*
yarn prepack
git diff

For the second location, Line 69 in packages/xdl/src/project/ExpSchema.ts:

1. Refactored the Cache out of the Api v1 class and into its own: XdlSchemaClient.
2. Checked it worked in a similar way to the first point, but this time checking that~/.expo/caches/ was repopulated by running expo start in an expo project. (Note: you have to delete ~/expo-cli/packages/xdl/caches/* before running this test to force the repopulation to happen from the endpoint instead of the local backup.)

[fson]

A better name for this would be ConfigurationSchemaClient. (The old method name xdlSchemaAsync was incorrect, or at least outdated by now.)

But since this "singleton" only has static properties and this is not Java, you might as well remove the class and have a function like getConfigurationSchemaAsync. (_schemaCaches can be a top-level variable in the module.) Please also move it to ExpSchema.ts, Utils only has some helper functions that are not really bound to our APIs or anything like that.