Update ietf-layer0-types-ext.yang

[italobusi]

Added definitions that were defined in layer0-types and removed from layer0-types during WG LC with the intention to further develop their definitions in the layer0-types-ext

See: https://github.com/ietf-ccamp-wg/draft-ietf-ccamp-layer0-types/commit/10acf682d70a734de21a81868f4f13a2b9c1a8f3

These definitions have not been copied:

• typedef operational-mode: superseded by the latest version of l0-types-ext
• typedef frequency-thz: superseded by the latest version of l0-types-ext
• typedef frequency-ghz: superseded by the latest version of l0-types-ext
• identity layer0-node-type: WG LC discussion has clarified that it is not needed
• grouping wson-path-bandwidth: WG LC discussion has clarified that it is not needed
• grouping wson-link-bandwidth: WG LC discussion has clarified that it is not needed
• grouping flexi-grid-node-attributes: WG LC discussion has clarified that it is not needed
• grouping flexi-grid-path-bandwidth: WG LC discussion has clarified that it is not needed
• grouping flexi-grid-link-bandwidth: WG LC discussion has clarified that it is not needed

These definitions have been copied:

• identity wavelength-assignment: it may be useful for path computation or tunnel models
• identity term-type: see open issue #14
• identity layer0-bandwidth-type: see open issue #15
• identity fec-type: see open issue #16

[italobusi]

@sergiobelotti @italobusi : update the name of layer0-bandwidth-type as otu-type @ggalimba56 @egriseri : to provide a list of needed FEC type @sergiobelotti @italobusi : based on the list provided in the previous AP, update YANG accordingly