Extend transfer_syntax module and re-export inventory

[Enet4]

This makes a few quality of life improvements that make it easier to register new transfer syntax using the inventory-based registry.

• [encoding] re-export more things at crate root
  • AdapterFreeTransferSyntax and inventory crate (if inventory-registry feature is enabled.
• [transfer-syntax-registry] remove inventory as own dependency, use the inventory re-export from dicom-encoding instead
• [encoding] improve docs on transfer_syntax module
• [encoding] extend transfer_syntax with submit_ele_transfer_syntax!
  • like submit_transfer_syntax!, but specifically for explicit VR little endian.