Rework how to fetch the main preparser, remove *vlc_MetadataRequest*
This MR aims to remove the following API:
- vlc_MetadataRequest
- vlc_MetadataRequestCancel
- libvlc_MetadataRequest
- libvlc_MetadataRequestCancel
That were just confusing helpers on top of the main preparser.
After this MR, it will be possible to:
- Lazy initialize the main preparser (since it's not used in all use case)
- Rework the preparser Push API (creation arguments)