Support passing in literal numbers on uint64 and int64 types

Accepts both string and number when the API expects uint64 or int64.

This solution allows regular JS integers (2^53-1) but can also take a string for larger numbers if needed. 

See googleapis/nodejs-bigquery#1350 for specific discussion on issue.

src/converter.js

@@ -34,6 +34,9 @@ class Converter {
     if (schema.type === 'integer') {
       return 'number';
     }
+    if (schema.type === 'string' && (schema.format === 'uint64' || schema.format === 'int64')) {
+      return 'number | string';
+    }
     if (schema.type === 'array') {
       return this.toArray(schema.items, schema.additionalItems);
     }