Add support for proto3 optional

[Tim-Zhang]

• compiler: Signal that ttrpc-compiler supports proto3 optionals
• ttrpc: bump protobuf to 2.27.1
• ttrpc-codegen: set optional as an allowd label
• example: update for optional testing

[codecov[bot]]

Codecov Report

Merging #156 (8ad8f2f) into master (4f1e5f1) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #156   +/-   ##
=======================================
  Coverage   26.00%   26.00%           
=======================================
  Files          16       16           
  Lines        2415     2415           
=======================================
  Hits          628      628           
  Misses       1787     1787           

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[Tim-Zhang]

Wait for pr https://github.com/stepancheg/rust-protobuf/pull/649