Add `itemName` for paged operations in generated openapi spec

[catalinaperalta]

When defining a paged operation in TypeSpec, folks who are still relying on the generated openapi spec need the itemName field to be part of the x-ms-pageable data. typespec-autorest should output this information.

Playground: example

Current output:

        "x-ms-pageable": {
          "nextLinkName": "nextLink"
        }

Expected output:

        "x-ms-pageable": {
          "itemName": "items",
          "nextLinkName": "nextLink"
        }

[catalinaperalta]

• @minhanh-phan since the App Config v-team needs support for this (example in app config)

[timotheeguerin]

Smaller repro