Fix cestory build

[bertho-zero]

Without the --experimental_allow_proto3_optional argument I can't build the cess node, I get this error:

error: failed to run custom build command for cestory-api v0.1.0 (/home/bertho/dev/cess/crates/cestory/api)

Caused by:
  process didn't exit successfully: /home/bertho/dev/cess/target/release/build/cestory-api-b00a4b4248836699/build-script-build (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=ceseal_rpc.proto
  cargo:rerun-if-changed=proto

  --- stderr
  thread 'main' panicked at crates/cestory/api/build.rs:34:10:
  called Result::unwrap() on an Err value: Custom { kind: Other, error: "protoc failed: ceseal_rpc.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.\n" }
  note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...