Summary:
Add load_latest_options() to C api.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10152

Test Plan:
Extend the existing c_test by reopening db using the latest options file
at different parts of the test.

Reviewed By: akankshamahajan15

Differential Revision: D37305225

Pulled By: ajkr

fbshipit-source-id: 8b3bab73f56fa6fcbdba45aae393145d007b3962