[roll] Roll fuchsia [zx_types][rs] Clean up padding fields.

All padding fields should:

* be private so that callers don't depend on their contents
* be named paddingN with N for the field padding field starting at 1 and
  incrementing by 1 for each subsequent padding field
* have the type [PadByte; N]

Also changes PadByte to be:

* private to ensure fields using it are hidden from callers
* repr(transparent) to guarantee ABI-compat with a single byte

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1135008
Original-Revision: 7f5dc3570dd5443078d50f8dfc9f23191769f64f
GitOrigin-RevId: b6c782a4b80a2dfe68f07efce9e3a2fba5579c23
Change-Id: Ieb97659236645c5cdaefb76c4b0a5c98c207362f

Author: Adam Perry

stem

@@ -11,7 +11,7 @@
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