Run fuzzing in CI

A few issues have been raised by fuzzing the parser (#334, #271, #233, #192), but so far it's only been done manually. The OSS-Fuzz project lets open source projects use their infrastructure to run fuzzing in CI: https://google.github.io/oss-fuzz/getting-started/continuous-integration/. For an example integration, see the grpc-gateway: grpc-ecosystem/grpc-gateway#2405. It appears the protoreflect project already exists in the projects directory, so it should just be a matter of adding a new CI job like for the grpc-gateway.

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