Provide hook_brightcove_cache_set_alter()

The cache TTL used by brightcove_cache_set() is established like so:

    $cache_settings = variable_get('brightcove_cache_db', []);
    $cache_time = isset($cache_settings['cache_time']) ? $cache_settings['cache_time'] : 600;

In practice it can be useful if different types of value (recognisable by their cache ID patterns) can be cached for different durations. An alter hook facilitates this nicely.