jsonschema: resolve remote references

Support remote references.
Most of the machinery was in place; we just had to define the
right loader, copy files from the test repo, and download
the meta-schema files from json-schema.org.

Change-Id: Ib588200ef0fa483641b9b53f9904f53b39dc4b82
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/testdata/remotes/draft2020-12/nested/foo-ref-string.json

@@ -0,0 +1,7 @@
+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "type": "object",
+    "properties": {
+        "foo": {"$ref": "string.json"}
+    }
+}