meta: expose API to register and create custom meta

Custom meta is backed by a GstStructure, and does not require
that users of the API expose their GstMeta implementation as
public API for other components to make use of it.

In addition, it provides a simpler interface by ignoring the
impl vs. api distinction that the regular API exposes.

This new API is meant to be the meta counterpart to custom events
and messages, and to be more convenient than the lower-level API
when the absolute best performance isn't a requirement.

Part-of: <gstreamer/gstreamer!609>
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