Adds support for storage engine customization of indexes to be specified at the collection level. Applies to all indexes created on a collection, including the _id index.

The WiredTiger integration layer uses this to produce a creation string with the following order of preference:

1. index-specific options specified to createIndex().
2. collection-wide options specified as "indexOptionDefaults" to createCollection().
3. system-wide options specified by --wiredTigerIndexConfigString.

The strings are concatenated so keys specified at the system-level and not overridden at the collection-level will be applied to the index.