Support Protobuf 30+ (Protobuf C++ 6.x)

[cho-m]

With Protobuf C++ 5.x (Protobuf 26 - 29) end of support on 31 Mar 2026¹, it would be nice to update to Protobuf C++ 6.x (Protobuf 30 - 33).

protobuf-javascript already added support in protocolbuffers/protobuf-javascript@eb785a9 which can be used as reference.

The main incompatibility is from various APIs now returning an absl::string_view rather than std::string. For this particular change:

• The naive/quick fix is to just find-replace everything impacted and wrap with std::string(...).
• A more performant option is to directly use the string_view and functions provided by Abseil - https://abseil.io/docs/cpp/guides/strings

Footnotes

1. https://protobuf.dev/support/version-support/#cpp ↩