Allow sanitize_meta() to filter the meta_value regardless of meta_key

[mattkeys]

I have created a couple of custom DB tables to handle storing a custom data type, and the meta for that data type. I modeled my meta table off of the WP postmeta/usermeta/etc standards so that I can use get_metadata and update_metadata functions and leverage all of their abilities.

Part of the update_metadata function is a call to sanitize_meta which lets you add filters in to sanitize the data in the format of: "sanitize_{$meta_type}_meta_{$meta_key}".

For the meta that I will be storing, I would like to apply a filter to ALL meta for my $meta_type, in the format of: "sanitize_{$meta_type}_meta".

[mattkeys]

Adding two new filters to sanitize_meta to allow for filtering of all meta values regardless of meta key