fix(grpc): Fix error "proto3 disallow 'optional' label"

[yixinguo-ssc]

Summary

Fix Issue 14421
grpc-gateway fails to load proto file with message "proto3 disallow 'optional' label"

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[14421](#14421)

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[StarlightIbuki]

With 1 more approval I will add change log for this

[Oyami-Srk]

LGTM, Thanks for your contribution! 🚀

[yixinguo-ssc]

With 1 more approval I will add change log for this

@StarlightIbuki PR now has 2 approvals. Could you add change log please?

[StarlightIbuki]

@yixinguo-ssc I don't have write access to your branch. You could add a changelog file to changelog/unreleased/kong/fix-missing-grpc-optional-support.yml with content below:

message: Fixed an issue where gRPC plugins lack support for "optional".
scope: Plugins
type: bugfix