[WebGPU][Kotlin] Use addins in dawn_kotlin.json

- Replace the bespoke customize_api block with the generic addins
  mechanism from https://dawn-review.googlesource.com/c/dawn/+/332056,
  matching dawn_native.json.
- Lets include_*/kotlin_name read attributes directly off the parsed
  Python objects instead of walking five parallel lookup dicts, and
  drops the obj/structure_name plumbing from include_method and
  kotlin_record_members.
- exclude_members becomes per-member "omitted" addins. Among record
  members, "omitted" is asserted to only appear on structure members,
  since omitting a function/method argument would silently pass a
  zero-initialized value to the native call.
- additional_members are now appended to struct.members
  (kotlin_only=True) so templates see a single member list; the
  converter analysis skips kotlin_only members accordingly.
- Document the dawn_kotlin.json schema and addin vocabulary in
  docs/dawn/codegen.md.
- No change to generated Kotlin/JNI output (verified by diffing the
  output of the kotlin generator target against origin/main, and
  running connectedDebugAndroidTest on device).

Suggested by lokokung@ in
https://dawn-review.googlesource.com/c/dawn/+/343975.

Bug: 568205909, 462825233
Change-Id: I2abdc9ef4f8f5c76dd636ca1b99caaba6b4c9b3c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/347535
Commit-Queue: Nikita Gupta <nikigupta@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>