Run fuzzing in CI

[johanbrandhorst]

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.

[jhump]

but so far it's only been done manually.

That's not entirely true. The Fuzzing is already done continuously. In fact it reported numerous issues that were all caused by one issue (fixed in #412).

It's just not part of the PR checks. (And, honestly, neither are the Circle CI checks for any contributed PRs -- for security reasons, Circle defaults to not running jobs if the source repo is a fork.)