Reland "pw_protobuf: Only generate `kMaxEncodedSizeBytes` in constrained structs"

This is a reland of commit 3cc0be95c4f3cb4e3bd458677b915cfe9c28619f

Original change's description:
> pw_protobuf: Only generate `kMaxEncodedSizeBytes` in constrained structs
>
> This prevents the PWPB code generator from outputting the
> `kMaxEncodedSizeBytes` constant unless every field within a message
> struct has a known maximum size.
>
> Fixes: 379868242
> Change-Id: Ia7eed0f95038f889262b2e49c215e73f363162a7
> Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/259334
> Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
> Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
> Docs-Not-Needed: Alexei Frolov <frolv@google.com>
> Reviewed-by: Wyatt Hepler <hepler@google.com>
> Tests-Not-Needed: Alexei Frolov <frolv@google.com>
> Commit-Queue: Alexei Frolov <frolv@google.com>

Change-Id: Ibcac8d545dcb00d0f368471a20113d4f011e57ba
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/302032
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
Tests-Not-Needed: Alexei Frolov <frolv@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Docs-Not-Needed: Alexei Frolov <frolv@google.com>