[Extensions] Enum modernisation for platform_keys.idl

This CL updates the codegen attribute for the enum types generated by
platform_keys.idl. This attribute generates enum class types with
kCamelCase key names.

Note: This CL is purely mechanical and should have no effect on existing
behaviour.

Bug: 1421546
Change-Id: Ica31c5fddc43101293f9a3b5b523e15cb12e7795
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4994295
Commit-Queue: Claudio DeSouza <cdesouza@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1219310}